[gimp] themes: toggle buttons were not styled at all when toggled.

[Jehan <jehanp src gnome org>]

commit 6c0c2f15c4460e3ad2dbcb0ed576ddf375001566
Author: Jehan <jehan girinstud io>
Date:   Thu Oct 13 15:55:39 2022 +0200

    themes: toggle buttons were not styled at all when toggled.

 themes/Default/common.css | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/themes/Default/common.css b/themes/Default/common.css
index 903c538609..8d8b3fb540 100644
--- a/themes/Default/common.css
+++ b/themes/Default/common.css
@@ -271,6 +271,10 @@ button.suggested-action {
   border: 1px dotted shade(@fg-color, 0.5);
 }
 
+button:checked {
+  background: @selected-color;
+}
+
 /* Spinbuttons: there are two kinds:
  * spinbutton, spinbutton button
  *   e.g. the "width" field in the New Image dialog.