feat: Visually distinguish captions from floating text

Resolves #224

Author: oodamien

src/css/vars.css

@@ -48,6 +48,7 @@
   --scrollbar-thumb-color: silver;
   --mark-background-color: #80ea6e;
   --selected-background-color: #191e1e;
+  --caption-font-style: italic;
 
   /* Layout */
   --layout-banner-height: 80px;