themes: draw a wider border around GimpFgBgColor's active color

Not quite optimal but at least some indication of the active color,
or at least of the fact that there are two different states.

themes/System/gimp.css

@@ -57,6 +57,10 @@ tab GimpFgBgView {
     padding: 6px 6px 6px 6px;
 }
 
+GimpFgBgEditor:active {
+    border-width: 2px 2px 2px 2px;
+}
+
 GimpDock {
     font-size: smaller;
 }