ENH: add optional extra_classes in version-switcher

docs/user_guide/configuring.rst

@@ -386,6 +386,9 @@ each can have the following fields:
   switcher.
 - ``name``: an optional name to display in the switcher dropdown instead of the
   version string (e.g., "latest", "stable", "dev", etc).
+- ``extra_classes``: an optional list of classes to add to the switcher
+  button for a given version (e.g., ``["dev", "rc"]``). These classes are only
+  added when the version is active.
 - ``url``: an optional URL. If provided, it links the version to ``url``
   instead of ``switcher['url_template']``.
 
@@ -398,6 +401,10 @@ Here is an example JSON file:
             "name": "v2.1 (stable)",
             "version": "2.1"
         },
+        {
+            "version": "2.1rc1",
+            "extra_classes": ["dev", "rc"]
+        },
         {
             "version": "2.0"
         },

src/pydata_sphinx_theme/theme/pydata_sphinx_theme/_templates/version-switcher.html

@@ -74,6 +74,10 @@
             if (entry.version == "{{ theme_switcher.get('version_match') }}") {
                 node.classList.add("active");
                 $("#version_switcher_button").text(entry.name);
+                // Add extra class to the button for the selected entry
+                if ("extra_classes" in entry) {
+                    $("#version_switcher_menu").classList.add(...entry.extra_classes);
+                }
             }
         });
     });