diff --git a/src/components/ThemeProvider/README.md b/src/components/ThemeProvider/README.md
new file mode 100644
index 0000000..1229f9f
--- /dev/null
+++ b/src/components/ThemeProvider/README.md
@@ -0,0 +1,163 @@
+### How It Works
+
+`ThemeProvider` manages the active theme by generating CSS custom properties into a `<style>` block in `<head>` and switching themes via a `data-theme` attribute on `<html>`.
+
+At registration time, each theme's CSS variables are pre-computed and cached.
+`getThemeStyleSheet()` serializes all registered themes into a single CSS block scoped by `:root[data-theme="<key>"]`.
+Switching themes is a single `setAttribute` call rather than N `setProperty` calls.
+
+The active theme key is persisted to `localStorage` under `THEME_STORAGE_KEY` (`"cc-theme"`).
+A blocking preflight script (`injectThemePreflight()`) can be dropped into `<head>` to apply the saved theme before first paint.
+This avoids a flash of unstyled content on first paint.
+
+### Basic Usage
+
+Wrap your app in `ThemeProvider`.
+No other setup is needed for React apps.
+The stylesheet is injected automatically on mount.
+
+```tsx
+import { ThemeProvider } from '@ianpaschal/combat-command-components';
+
+export const App = () => (
+  <ThemeProvider>
+    <YourApp />
+  </ThemeProvider>
+);
+```
+
+To lock a specific theme (e.g. in a preview or demo), pass the `theme` prop:
+
+```tsx
+<ThemeProvider theme="dark">
+  <YourApp />
+</ThemeProvider>
+```
+
+### Built-In Themes
+
+| Key | Display Name | Dark |
+|---|---|---|
+| `light` | Light | No |
+| `dark` | Dark | Yes |
+| `daybreak` | Daybreak | No |
+| `midnight` | Midnight | Yes |
+
+`SYSTEM_THEME_KEY` (`"__system"`) resolves to `dark` or `light` based on `prefers-color-scheme`.
+It is the default when no preference is stored.
+
+### Accessing the Theme in a Component
+
+```tsx
+import { useThemeManager } from '@ianpaschal/combat-command-components';
+
+const { key, theme, options, setTheme } = useThemeManager();
+```
+
+| Property | Type | Description |
+|---|---|---|
+| `key` | `string` | The active key, including `"__system"` if no explicit choice was made. |
+| `theme` | `Theme` | The resolved `Theme` object. |
+| `options` | `SelectOption[]` | All registered themes plus the system option, ready for a `<Select>`. |
+| `setTheme` | `(key: string) => void` | Updates the active theme and persists it to localStorage. |
+
+### Registering a Custom Theme
+
+Call `registerTheme` before the app mounts.
+Each registered theme is merged on top of a parent (defaults to `light`).
+
+```ts
+import { registerTheme } from '@ianpaschal/combat-command-components';
+
+registerTheme('branded', {
+  displayName: 'Branded',
+  dark: false,
+  surface: {
+    page: { bg: '#f0e8ff' },
+    card: { bg: '#ffffff', border: '#d8c8f0' },
+  },
+  colors: {
+    accent: { bg: '#7c3aed', text: '#ffffff', focus: '#7c3aed' },
+  },
+});
+```
+
+Custom themes registered before `getThemeStyleSheet()` is first called are included in the generated stylesheet.
+
+### SSR / Static Sites (e.g. Astro)
+
+For server-rendered pages, inject the stylesheet and preflight script into `<head>` before any content renders:
+
+```astro
+---
+import {
+  getThemeStyleSheet,
+  injectThemePreflight,
+} from '@ianpaschal/combat-command-components';
+
+const themeCSS = getThemeStyleSheet();
+---
+<head>
+  <!-- Inject CSS vars for all themes, scoped to :root[data-theme]. -->
+  <style is:inline set:html={themeCSS}></style>
+  <!-- Blocking script: reads localStorage and sets data-theme before first paint. -->
+  <script is:inline set:html={injectThemePreflight()}></script>
+</head>
+```
+
+The `<style>` tag carries a `data-theme-vars` attribute so the client-side `ThemeProvider` won't inject a duplicate.
+
+### API Reference
+
+#### `ThemeProvider`
+
+| Prop | Type | Default | Description |
+|---|---|---|---|
+| `theme` | `string` | - | Locks the active theme; overrides user selection and localStorage. |
+| `children` | `ReactNode` | - | |
+
+#### `getThemeStyleSheet(): string`
+
+Serializes all registered themes' CSS variables into a single CSS string scoped by `:root[data-theme="<key>"]`.
+In a browser context, also injects the result as a `<style data-theme-vars>` element in `<head>` (idempotent).
+Returns the CSS string.
+
+#### `injectThemePreflight(defaults?): string`
+
+Returns a self-executing script string that reads `localStorage.getItem("cc-theme")` and sets `data-theme` on `<html>` before first paint.
+If no key is stored or the stored key is `"__system"`, it falls back to `prefers-color-scheme`, mapping to the resolved theme keys.
+
+`defaults` is an optional object that overrides the theme keys used when the stored value is `"__system"`:
+
+| Property | Type | Default | Description |
+|---|---|---|---|
+| `dark` | `string` | `"dark"` | The theme key applied when `prefers-color-scheme: dark` matches. |
+| `light` | `string` | `"light"` | The theme key applied otherwise. |
+
+When omitted, `"dark"` and `"light"` are used.
+
+```ts
+// Use built-in keys (dark → "dark", light → "light")
+injectThemePreflight()
+
+// Map system dark/light to custom registered keys
+injectThemePreflight({ dark: 'midnight', light: 'daybreak' })
+```
+
+Drop the returned string into a blocking `<script>` in `<head>`.
+
+#### `registerTheme(key, theme, parentKey?)`
+
+Registers a new theme or overrides an existing one.
+`theme` is deep-merged onto `parentKey` (Default: `"light"`).
+CSS variables are computed and cached immediately.
+
+#### `THEME_STORAGE_KEY`
+
+The localStorage key used to persist the active theme choice (`"cc-theme"`).
+Use this if you need to read or write the preference outside of `ThemeProvider`.
+
+#### `SYSTEM_THEME_KEY`
+
+The sentinel value (`"__system"`) that resolves to `"light"` or `"dark"` based on `prefers-color-scheme`.
+Passed to `setTheme` to restore automatic OS-based switching.
diff --git a/src/components/ThemeProvider/ThemeProvider.constants.ts b/src/components/ThemeProvider/ThemeProvider.constants.ts
new file mode 100644
index 0000000..5fe2f23
--- /dev/null
+++ b/src/components/ThemeProvider/ThemeProvider.constants.ts
@@ -0,0 +1,10 @@
+export const THEME_STORAGE_KEY = 'cc-theme';
+export const SYSTEM_THEME_KEY = '__system';
+
+const VALID_THEME_KEY = /^[a-zA-Z0-9_-]+$/;
+
+export const validateKey = (key: string, context: string): void => {
+  if (!VALID_THEME_KEY.test(key)) {
+    throw new Error(`Invalid theme key "${key}" (${context}). Keys must contain only letters, numbers, hyphens, and underscores.`);
+  }
+};
diff --git a/src/components/ThemeProvider/ThemeProvider.hooks.ts b/src/components/ThemeProvider/ThemeProvider.hooks.ts
index 6417390..f4ac8e9 100644
--- a/src/components/ThemeProvider/ThemeProvider.hooks.ts
+++ b/src/components/ThemeProvider/ThemeProvider.hooks.ts
@@ -7,16 +7,31 @@ import {
 import { useStore } from '@tanstack/react-store';
 
 import { light } from './themes/light';
+import { SYSTEM_THEME_KEY } from './ThemeProvider.constants';
 import { themeContext } from './ThemeProvider.context';
 import { themeStore } from './ThemeProvider.store';
 import { Theme } from './ThemeProvider.types';
-import { buildThemeVars } from './ThemeProvider.utils';
 
-export const SYSTEM_THEME_KEY = '__system';
+export const useIsomorphicLayoutEffect = typeof window !== 'undefined' ? useLayoutEffect : useEffect;
 
-export const useResolvedTheme = (activeKey: string): Theme => {
+/**
+ * Resolves an active theme key to a `Theme` object and a concrete registry key.
+ * Handles `SYSTEM_THEME_KEY` by mapping it to `"dark"` or `"light"` based on
+ * `prefers-color-scheme`, and subscribes to OS preference changes.
+ *
+ * @param activeKey - The active theme key, which may be `SYSTEM_THEME_KEY`.
+ * @returns The resolved `Theme` object and the concrete key (never
+ *   `SYSTEM_THEME_KEY`).
+ */
+export const useResolvedTheme = (activeKey: string): { theme: Theme; resolvedKey: string } => {
   const registry = useStore(themeStore);
-  const [isDark, setIsDark] = useState(() => window.matchMedia('(prefers-color-scheme: dark)').matches);
+  const [isDark, setIsDark] = useState(() => (
+    typeof window !== 'undefined' && window.matchMedia('(prefers-color-scheme: dark)').matches
+  ));
+
+  useIsomorphicLayoutEffect(() => {
+    setIsDark(window.matchMedia('(prefers-color-scheme: dark)').matches);
+  }, []);
 
   useEffect(() => {
     const mq = window.matchMedia('(prefers-color-scheme: dark)');
@@ -26,18 +41,20 @@ export const useResolvedTheme = (activeKey: string): Theme => {
   }, []);
 
   if (activeKey === SYSTEM_THEME_KEY) {
-    return isDark ? (registry['dark'] ?? registry['light'] ?? light) : (registry['light'] ?? light);
+    const resolvedKey = isDark ? 'dark' : 'light';
+    return {
+      theme: registry[resolvedKey]?.theme ?? registry['light']?.theme ?? light,
+      resolvedKey,
+    };
   }
-  return registry[activeKey] ?? registry['light'] ?? light;
+  return {
+    theme: registry[activeKey]?.theme ?? registry['light']?.theme ?? light,
+    resolvedKey: activeKey,
+  };
 };
 
+/**
+ * Returns the theme context value from the nearest `ThemeProvider`, exposing
+ * the active key, resolved `Theme` object, available options, and `setTheme`.
+ */
 export const useThemeManager = () => useContext(themeContext);
-
-export const useThemeVars = (theme: Theme): void => {
-  useLayoutEffect(() => {
-    const vars = buildThemeVars(theme);
-    for (const [key, value] of Object.entries(vars)) {
-      document.body.style.setProperty(key, value);
-    }
-  }, [theme]);
-};
diff --git a/src/components/ThemeProvider/ThemeProvider.store.ts b/src/components/ThemeProvider/ThemeProvider.store.ts
index f8f2d7b..998f305 100644
--- a/src/components/ThemeProvider/ThemeProvider.store.ts
+++ b/src/components/ThemeProvider/ThemeProvider.store.ts
@@ -6,27 +6,139 @@ import { daybreak } from './themes/daybreak';
 import { light } from './themes/light';
 import { midnight } from './themes/midnight';
 import { DeepPartial } from '../../types';
-import { Theme } from './ThemeProvider.types';
+import {
+  SYSTEM_THEME_KEY,
+  THEME_STORAGE_KEY,
+  validateKey,
+} from './ThemeProvider.constants';
+import { Theme, ThemeRegistryEntry } from './ThemeProvider.types';
+import { buildThemeVars } from './ThemeProvider.utils';
 
-export const themeStore = new Store<Record<string, Theme>>({
-  light,
-  dark,
-  daybreak,
-  midnight,
+const makeEntry = (theme: Theme): ThemeRegistryEntry => ({
+  theme,
+  vars: buildThemeVars(theme),
 });
 
-export const registerTheme = (key: string, theme: DeepPartial<Theme>, parentKey?: string): void => {
+export const themeStore = new Store<Record<string, ThemeRegistryEntry>>({
+  light: makeEntry(light),
+  dark: makeEntry(dark),
+  daybreak: makeEntry(daybreak),
+  midnight: makeEntry(midnight),
+});
+
+/**
+ * Registers a new theme or overrides an existing one. The provided theme is
+ * deep-merged onto the parent (defaults to `light`). CSS variables are computed
+ * and cached immediately.
+ *
+ * @param key - Unique key used to identify and activate the theme.
+ * @param theme - Partial theme object; missing values are inherited from the
+ *   parent.
+ * @param parentKey - Key of the theme to inherit from. Defaults to `"light"`.
+ */
+export const registerTheme = (
+  key: string,
+  theme: DeepPartial<Theme>,
+  parentKey?: string,
+): void => {
+  validateKey(key, 'registerTheme');
   themeStore.setState((state) => {
-    const parent = parentKey ? (state[parentKey] ?? light) : light;
-    return { ...state, [key]: deepmerge(parent, theme as Theme) };
+    if (parentKey && !state[parentKey]) {
+      console.warn(`registerTheme: parent key "${parentKey}" not found for theme "${key}". Falling back to "light".`);
+    }
+    const parent = parentKey ? (state[parentKey]?.theme ?? light) : light;
+    const merged = deepmerge(parent, theme as Theme);
+    return { ...state, [key]: makeEntry(merged) };
   });
 };
 
+/**
+ * Returns the resolved `Theme` object for the given key. Falls back to `light`
+ * and logs a warning if the key is not registered.
+ *
+ * @param key - Key of the registered theme to retrieve.
+ */
 export const getRegisteredTheme = (key: string): Theme => {
-  const theme = themeStore.state[key];
-  if (!theme) {
+  const entry = themeStore.state[key];
+  if (!entry) {
     console.warn(`Could not find a theme with key ${key}. Will use 'light' instead.`);
     return light;
   }
-  return theme;
+  return entry.theme;
+};
+
+/**
+ * Serializes all registered themes' CSS variables into a single stylesheet
+ * string, with each theme scoped to `:root[data-theme="<key>"]`. In a browser
+ * context, also injects or updates a `<style data-theme-vars>` element in
+ * `<head>` (idempotent).
+ *
+ * @returns The generated CSS string.
+ */
+export const getThemeStyleSheet = (): string => {
+  const css = Object.entries(themeStore.state).map(([key, { vars }]) => {
+    const safeKey = key.replace(/\\/g, '\\\\').replace(/"/g, '\\"');
+    const declarations = Object.entries(vars).map(([k, v]) => (
+      `  ${k}: ${v};`
+    )).join('\n');
+    return `:root[data-theme="${safeKey}"] {\n${declarations}\n}`;
+  }).join('\n\n');
+
+  if (typeof document !== 'undefined') {
+    const existing = document.querySelector('style[data-theme-vars]');
+    if (existing) {
+      existing.textContent = css;
+    } else {
+      const style = document.createElement('style');
+      style.setAttribute('data-theme-vars', '');
+      style.textContent = css;
+      document.head.appendChild(style);
+    }
+  }
+
+  return css;
+};
+
+/**
+ * Returns a self-executing script string that reads `localStorage` and sets
+ * `data-theme` on `<html>` before first paint, preventing a flash of unstyled
+ * content. Also installs a `MutationObserver` to re-apply the theme if
+ * `data-theme` is removed (e.g. during Astro page transitions). Drop the
+ * returned string into a blocking `<script>` in `<head>`.
+ *
+ * @param defaults - Optional overrides for the theme keys used when the stored
+ *   value is `SYSTEM_THEME_KEY`. Defaults to `{ dark: "dark", light: "light" }`.
+ */
+export const injectThemePreflight = (
+  defaults?: { dark?: string; light?: string },
+): string => {
+  const dark = defaults?.dark ?? 'dark';
+  validateKey(dark, 'injectThemePreflight defaults.dark');
+  const light = defaults?.light ?? 'light';
+  validateKey(light, 'injectThemePreflight defaults.light');
+  return `
+    (() => {
+      const applyTheme = () => {
+        var key = '${SYSTEM_THEME_KEY}';
+        try {
+          key = localStorage.getItem('${THEME_STORAGE_KEY}') || '${SYSTEM_THEME_KEY}';
+        } catch(e) {}
+        if (key === '${SYSTEM_THEME_KEY}') {
+          key = window.matchMedia('(prefers-color-scheme: dark)').matches ? '${dark}' : '${light}';
+        }
+        document.documentElement.setAttribute('data-theme', key);
+      };
+      applyTheme();
+      new MutationObserver((mutations) => {
+        mutations.forEach((mutation) => {
+          if (mutation.attributeName === 'data-theme' && !document.documentElement.getAttribute('data-theme')) {
+            applyTheme();
+          }
+        });
+      }).observe(document.documentElement, {
+        attributes: true,
+        attributeFilter: ['data-theme'],
+      });
+    })()
+  `;
 };
diff --git a/src/components/ThemeProvider/ThemeProvider.tsx b/src/components/ThemeProvider/ThemeProvider.tsx
index 893d73c..d0387fd 100644
--- a/src/components/ThemeProvider/ThemeProvider.tsx
+++ b/src/components/ThemeProvider/ThemeProvider.tsx
@@ -5,13 +5,10 @@ import {
 } from 'react';
 import { useStore } from '@tanstack/react-store';
 
+import { SYSTEM_THEME_KEY, THEME_STORAGE_KEY } from './ThemeProvider.constants';
 import { ThemeContextProvider } from './ThemeProvider.context';
-import {
-  SYSTEM_THEME_KEY,
-  useResolvedTheme,
-  useThemeVars,
-} from './ThemeProvider.hooks';
-import { themeStore } from './ThemeProvider.store';
+import { useIsomorphicLayoutEffect, useResolvedTheme } from './ThemeProvider.hooks';
+import { getThemeStyleSheet, themeStore } from './ThemeProvider.store';
 
 export interface ThemeProviderProps {
 
@@ -24,22 +21,41 @@ export const ThemeProvider = ({
   theme: forcedTheme,
   children,
 }: ThemeProviderProps) => {
-  const [key, setKey] = useState(SYSTEM_THEME_KEY);
+  const [key, setKey] = useState(() => (
+    typeof window !== 'undefined' ? (localStorage.getItem(THEME_STORAGE_KEY) ?? SYSTEM_THEME_KEY) : SYSTEM_THEME_KEY
+  ));
   const activeKey = forcedTheme ?? key;
-  const theme = useResolvedTheme(activeKey);
+  const { theme, resolvedKey } = useResolvedTheme(activeKey);
   const registry = useStore(themeStore);
   const options = useMemo(() => [
     { value: SYSTEM_THEME_KEY, label: 'System' },
-    ...Object.entries(registry).map(([k, { displayName }]) => ({
+    ...Object.entries(registry).map(([k, { theme: { displayName } }]) => ({
       value: k,
       label: displayName,
     })),
   ], [registry]);
 
-  useThemeVars(theme);
+  useIsomorphicLayoutEffect(() => {
+    getThemeStyleSheet();
+  }, [registry]);
+
+  useIsomorphicLayoutEffect(() => {
+    document.documentElement.setAttribute('data-theme', resolvedKey);
+  }, [resolvedKey]);
+
+  const handleSetTheme = (newKey: string) => {
+    if (!forcedTheme) {
+      try {
+        localStorage.setItem(THEME_STORAGE_KEY, newKey);
+      } catch (e) {
+        console.error(e);
+      }
+      setKey(newKey);
+    }
+  };
 
   return (
-    <ThemeContextProvider value={{ key: activeKey, theme, options, setTheme: setKey }}>
+    <ThemeContextProvider value={{ key: activeKey, theme, options, setTheme: handleSetTheme }}>
       {children}
     </ThemeContextProvider>
   );
diff --git a/src/components/ThemeProvider/ThemeProvider.types.ts b/src/components/ThemeProvider/ThemeProvider.types.ts
index 113d744..798a481 100644
--- a/src/components/ThemeProvider/ThemeProvider.types.ts
+++ b/src/components/ThemeProvider/ThemeProvider.types.ts
@@ -1,5 +1,10 @@
 import { ThemeColor } from '../../types';
 
+export type ThemeRegistryEntry = {
+  theme: Theme;
+  vars: Record<string, string>;
+};
+
 export type Theme = {
   displayName: string;
   dark: boolean;
diff --git a/src/components/ThemeProvider/index.ts b/src/components/ThemeProvider/index.ts
index dc7958b..d7dc73c 100644
--- a/src/components/ThemeProvider/index.ts
+++ b/src/components/ThemeProvider/index.ts
@@ -2,11 +2,13 @@ export {
   ThemeProvider,
   type ThemeProviderProps,
 } from './ThemeProvider';
+export { SYSTEM_THEME_KEY, THEME_STORAGE_KEY } from './ThemeProvider.constants';
+export { useThemeManager } from './ThemeProvider.hooks';
 export {
-  SYSTEM_THEME_KEY,
-  useThemeManager,
-} from './ThemeProvider.hooks';
-export { registerTheme } from './ThemeProvider.store';
+  getThemeStyleSheet,
+  injectThemePreflight,
+  registerTheme,
+} from './ThemeProvider.store';
 export type { Theme } from './ThemeProvider.types';
 export { dark } from './themes/dark';
 export { light } from './themes/light';
