fix(editor): improve dark theme contrast

[murongg]

Summary

• make Markdown markers and CodeMirror selections readable in source and preview modes across built-in dark themes
• add theme-aware Markdown metadata and JSON token highlighting instead of CodeMirror's light-theme fallback colors
• raise low-contrast secondary text tokens in Dark, One Dark, and One Dark Pro, including independently selected editor paper themes
• cover the affected syntax, selection, and theme combinations with regression tests

Why

The v2.1.0 syntax-muted styling blended structural Markdown markers too close to dark editor backgrounds. CodeMirror also applied light-theme syntax and selection fallbacks to source metadata and the custom headers JSON editor. One Dark's secondary text tokens were additionally too dim for settings descriptions and editor annotations.

Closes #601

Validation

• pnpm --filter @markra/app test — 129 files and 1458 tests passed
• pnpm --filter @markra/editor test — 33 files and 357 tests passed
• pnpm --filter @markra/editor typecheck:test — passed
• pnpm --filter @markra/app typecheck:test — passed
• pnpm --filter @markra/web build — passed
• git diff --check — passed
• contrast spot checks: Dark secondary text 4.52:1; One Dark secondary text 4.71:1; Solarized Dark markers 5.61:1

Risk

• Medium-low: the behavior is limited to editor highlighting, selection colors, and dark-theme text tokens. One Dark secondary text is intentionally brighter, so muted UI text will have less visual separation from primary text.

Screenshots

• Before: see [Bug]: 深色模式下源码模式语法符号与背景对比度过低，无法看清 #601.
• After: not captured in the automated environment; token rendering, selector precedence, and independent app/editor theme combinations are covered by regression tests.

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Project	Deployment	Actions	Updated (UTC)
markra-web	Ready	Preview, Comment	Jul 29, 2026 4:08am