themes: Prevent system theme leak in dialogue frames

Resolves #14421
This patch provides a CSS rule for GtkFrame borders inside of
a dialogue (see Filmstrip plug-in GUI for an example) to prevent
a system theme leak from altering the color.

themes/Default/common.css

@@ -670,6 +670,11 @@ border {
   border: 0px;
 }
 
+/* The border around frames in dialogs */
+dialog .frame {
+    border: 1px solid @border-color;
+}
+
 /* The border around GimpOffsetArea frame in resize dialogs */
 .gimp-offset-area-frame {
   border: solid 2px @stronger-border-color;