Improve title legibility on the L08 dividers and mini-lecture subtitles

Four Lecture 8 section dividers use light-background images (the Jupiter, Earth and Mercury interiors and the seismic-ray diagram), so the white section title washed out under the navy overlay. Give the section-divider macro an optional overlay-opacity argument and raise it to 0.65 on those four dividers; every other deck keeps the 0.45 default.

The mini-lecture title slides set their subtitle in a near-white fill that reads poorly where the background image is light, as on the Tutorial 4 slide over the Earth-interior image. Set the subtitle to solid white and bold in the shared title-page template, so it stays legible on every mini-lecture.

Rebuild the Lecture 8 web PDF and the four mini-lecture web PDFs.

Author: timlichtenberg

book/_static/minilectures/minilecture01.pdf

@@ -26,7 +26,7 @@
 \setbeamercolor{framesubtitle}{fg=ipsAccent}
 
 \setbeamercolor{title}{fg=ipsBackground, bg=ipsPrimary}
-\setbeamercolor{subtitle}{fg=ipsLightFill}
+\setbeamercolor{subtitle}{fg=white}
 \setbeamercolor{author}{fg=ipsLightFill}
 \setbeamercolor{institute}{fg=ipsLightFill}
 \setbeamercolor{date}{fg=ipsLightFill}
@@ -63,7 +63,7 @@
 \renewcommand{\familydefault}{\sfdefault}
 
 \setbeamerfont{title}{size=\Large, series=\bfseries}
-\setbeamerfont{subtitle}{size=\normalsize}
+\setbeamerfont{subtitle}{size=\normalsize, series=\bfseries}
 \setbeamerfont{frametitle}{size=\large, series=\bfseries}
 \setbeamerfont{framesubtitle}{size=\small}
 \setbeamerfont{footline}{size=\tiny}
@@ -185,8 +185,10 @@
 }
 
 % ── Section divider slides ──────────────────────────────────
-% Set per-section background: \sectionimage{filename}{credit text}
-\newcommand{\sectionimage}[2]{\def\ips@secimg{#1}\def\ips@seccredit{#2}}
+% Set per-section background: \sectionimage[overlay opacity]{filename}{credit text}
+% The optional first argument darkens the navy overlay for light-background
+% images so the white title stays legible; it defaults to 0.45.
+\newcommand{\sectionimage}[3][0.45]{\def\ips@secimg{#2}\def\ips@seccredit{#3}\def\ips@secopacity{#1}}
 \sectionimage{}{}% default empty
 
 \AtBeginSection[]{%
@@ -197,7 +199,7 @@
         \fill[ipsPrimary] (current page.north west) rectangle (current page.south east);
       \else
         \ipscontain{\ips@secimg}
-        \fill[ipsPrimary, opacity=0.45] (current page.north west) rectangle (current page.south east);
+        \fill[ipsPrimary, opacity=\ips@secopacity] (current page.north west) rectangle (current page.south east);
       \fi
       % Section title
       \node[anchor=center] at (current page.center) {%

book/_static/minilectures/minilecture02.pdf

@@ -52,7 +52,7 @@
 
 
 % ════════════════════════════════════════════════════════════
-\sectionimage{seismic_waves}{Seismic P-wave paths through Earth. Credit: Wikimedia Commons, public domain}
+\sectionimage[0.65]{seismic_waves}{Seismic P-wave paths through Earth. Credit: Wikimedia Commons, public domain}
 \section{Probing planetary interiors}
 % ════════════════════════════════════════════════════════════
 
@@ -271,7 +271,7 @@ \section{Blackboard derivation: moment of inertia factor}
 
 
 % ════════════════════════════════════════════════════════════
-\sectionimage{jupiter_interior}{Jupiter's interior. Credit: NASA/JPL-Caltech/SwRI/John E. Connerney (PIA25062)}
+\sectionimage[0.65]{jupiter_interior}{Jupiter's interior. Credit: NASA/JPL-Caltech/SwRI/John E. Connerney (PIA25062)}
 \section{Equations of state}
 % ════════════════════════════════════════════════════════════
 
@@ -394,7 +394,7 @@ \section{Equations of state}
 
 
 % ════════════════════════════════════════════════════════════
-\sectionimage{earth_interior}{Earth's interior. Credit: NASA/JPL-Caltech/SwRI/John E. Connerney (PIA25063)}
+\sectionimage[0.65]{earth_interior}{Earth's interior. Credit: NASA/JPL-Caltech/SwRI/John E. Connerney (PIA25063)}
 \section{Earth's interior}
 % ════════════════════════════════════════════════════════════
 
@@ -642,7 +642,7 @@ \section{Mantle rheology \& convection}
 \breakslide
 
 % ════════════════════════════════════════════════════════════
-\sectionimage{earth_interior}{Earth's interior. Credit: NASA/JPL-Caltech/SwRI/John E. Connerney (PIA25063)}
+\sectionimage[0.65]{earth_interior}{Earth's interior. Credit: NASA/JPL-Caltech/SwRI/John E. Connerney (PIA25063)}
 \section{Phase transitions in the mantle}
 % ════════════════════════════════════════════════════════════
 
@@ -731,7 +731,7 @@ \section{Phase transitions in the mantle}
 
 
 % ════════════════════════════════════════════════════════════
-\sectionimage{mercury_interior}{Mercury's layered interior. After Margot et al.\ (2018)}
+\sectionimage[0.65]{mercury_interior}{Mercury's layered interior. After Margot et al.\ (2018)}
 \section{Comparative terrestrial interiors}
 % ════════════════════════════════════════════════════════════