[5.0.x] Refs #33690 -- Added missing data-theme selector to example in theming support docs.

Backport of 640283711e6c8d25ad0e3c97453cd178a3e4d6a6 from main

docs/ref/contrib/admin/index.txt

@@ -2807,7 +2807,7 @@ template override to your project:
 
     {% block extrastyle %}{{ block.super }}
     <style>
-    :root {
+    html[data-theme="light"], :root {
       --primary: #9774d5;
       --secondary: #785cab;
       --link-fg: #7c449b;