fix: give SVG images a white background in dark mode (#642)

We recently introduced dark mode. SVG images often have dark text on
transparent backgrounds, making them invisible when the page background
is dark.

As a workaround, add a white backdrop via CSS on SVGs (by file
extension). There may be better fixes here, but this works well in my
testing and requires no action from contributors (e.g. multiple images).

Before/after:
<img width="913" height="427" alt="image"
src="https://github.com/user-attachments/assets/9fd6fbf5-c609-45f8-a8a8-581e2889b651"
/>
<img width="913" height="427" alt="image"
src="https://github.com/user-attachments/assets/97bd46ba-2d02-4b05-9dd2-56e021040245"
/>

Author: mattkelly

_sass/modules/_dark-mode.scss

@@ -142,6 +142,15 @@
     border-radius: 4px;
     padding: 2px;
   }
+
+  // SVG diagrams commonly have dark text on transparent backgrounds, which
+  // becomes invisible against the dark page background. Give them a white
+  // backdrop. Targets only SVGs (diagrams), not photos (jpg/png).
+  article .content img[src$=".svg"] {
+    background-color: white;
+    border-radius: 4px;
+    padding: 4px;
+  }
 }
 
 // System preference: apply dark theme only when no explicit choice is saved.