themes: Define background-image for entry and switch

Resolves #12682
Some system themes add a gradient background-image to
text input and toggle widgets. On dark theme, this can
make the text unreadable if the background-image is a similar
color. This patch defines the background-image as none, to
prevent the system theme from leaking in.

themes/Default/common.css

@@ -342,6 +342,7 @@ GimpSamplePointEditor box.vertical {
 
 entry {
   background-color: @extreme-bg-color;
+  background-image: none;
   color: @fg-color;
 }
 
@@ -686,6 +687,7 @@ list row:hover {
 
 switch {
   background-color: @bg-color;
+  background-image: none;
   border: 1px solid @stronger-border-color;
 }