Leave selection colours to the browser, declare color-scheme instead

The unreadable selection came from Furo not declaring color-scheme, so
browsers used the OS dark-mode selection on light pages. Declaring the
active scheme lets the browser pick fitting native colours.

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

Author: honzajavorek

docs/_static/custom.css

@@ -32,25 +32,23 @@ body[data-theme="dark"] img.adaptive-image {
 }
 
 /*
-  Furo doesn't style text selection, so browsers use the system colour, which
-  is dark blue on a light page when the OS is in dark mode. Selected text,
-  including links, gets one high-contrast colour on a Pyvec tint. The sidebar
-  is slate in both modes, so it always uses the dark variant.
+  Furo doesn't declare color-scheme, so browsers follow the OS. On a light
+  page with the OS in dark mode, the native text selection was dark blue
+  and unreadable. Declare the active scheme, so the browser and OS pick
+  fitting native colours for selection, scrollbars, and form controls.
+  The sidebar is slate in both modes, so it's always dark.
 */
-::selection {
-  color: #000;
-  background: #c7e7f7;
+body {
+  color-scheme: light;
 }
 
-body[data-theme="dark"] ::selection,
-.sidebar-drawer ::selection {
-  color: #fff;
-  background: #3b4f5a;
+body[data-theme="dark"],
+.sidebar-drawer {
+  color-scheme: dark;
 }
 
 @media (prefers-color-scheme: dark) {
-  body:not([data-theme="light"]) ::selection {
-    color: #fff;
-    background: #3b4f5a;
+  body:not([data-theme="light"]) {
+    color-scheme: dark;
   }
 }