Fix dim illustrations in dark mode

Sphinx puts the image class on the wrapping link too, so the invert
filter applied twice and the illustrations came out dark grey.
Target only the img, so they match the logo.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: honzajavorek

docs/_static/custom.css

@@ -2,14 +2,17 @@
   Single-colour illustrations and the logo are drawn black. In dark mode,
   invert them to light grey (#ccc). Follows both Furo's toggle and, in its
   "auto" mode, the system setting, the same way Furo switches its own colours.
+
+  Sphinx puts the image's class on the wrapping link too, so the selector
+  must target only the img, or the filter would apply twice.
 */
-body[data-theme="dark"] .adaptive-image,
+body[data-theme="dark"] img.adaptive-image,
 body[data-theme="dark"] .sidebar-logo {
   filter: invert(80%);
 }
 
 @media (prefers-color-scheme: dark) {
-  body:not([data-theme="light"]) .adaptive-image,
+  body:not([data-theme="light"]) img.adaptive-image,
   body:not([data-theme="light"]) .sidebar-logo {
     filter: invert(80%);
   }