[gimp] themes: some fix to the new Light theme.

[Jehan <jehanp src gnome org>]

commit 6b3d69a3c50122a1d9143d03fd87b47f0df4cc1b
Author: Jehan <jehan girinstud io>
Date:   Fri Aug 19 19:54:59 2022 +0200

    themes: some fix to the new Light theme.
    
    These are issues which appeared to me only when GIMP was set to use the
    dark theme variant. In such a case, I guess we see quite well CSS rules
    coming from parent themes, such as the globally set system theme.
    
    * Remove any background image (which may come from the system theme?) on
      buttons.
    * The headerbar was dark and when in the background, it had a dark
      linear gradient (probably coming from my system theme). So make sure
      the background color is right as a general rule, and add back a
      (small) light gradient to indicate the background state.
    * GtkListBox had a dark background. We don't have a lot of usage for
      this widget yet, but a few (for instance in icon theme settings, or in
      the modifiers editor).
    * The GtkSwitch buttons were dark on dark. Set a light background. Also
      when the switch is ON, I add a bit of color, not too strong, but
      enough to indicate the checked status, because I find this widget
      design not so obvious. Since I don't think we use switch buttons
      anywhere else but in Preferences, this is probably not a big problem
      as it should not affect color perception when working on the canvas.

 themes/Light/gimp.css | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)
---
diff --git a/themes/Light/gimp.css b/themes/Light/gimp.css
index 87aef4e22f..b386255c9a 100644
--- a/themes/Light/gimp.css
+++ b/themes/Light/gimp.css
@@ -216,6 +216,10 @@ paned separator {
   color: black;
 }
 
+button {
+  background-image: none;
+}
+
 button:disabled {
   color: rgb(200,200,200);
 }
@@ -309,4 +313,33 @@ menuitem *:hover {
   background-color: rgb(235,235,235);
 }
 
+/* Header bar */
+
+headerbar {
+  background-color: rgb(235,235,235);
+  background-image: none;
+}
+
+headerbar:backdrop {
+  background-image: linear-gradient(to top, rgb(225,225,225), rgb(245,245,245));
+}
+
+/* GtkListBox widgets */
 
+list {
+   background-color: rgb(235,235,235);
+}
+
+/* GtkSwitch buttons */
+
+switch {
+   background-color: rgb(235,235,235);
+}
+
+/* The switch button is quite hard to understand without color IMO. So
+ * accept a bit of color here as an exception to indicate the checked
+ * state, even though this is globally a neutral gray theme.
+ */
+switch:checked {
+    background-color: rgb(200,200,255);
+}