Add a special style class for menu popover button

We want to have a non-osd popover in the button that is placed on OSD headerbar. This is something unusual that Adwaita does not have, so create a custom style that will use common values from user system theme.

css/styles.css

@@ -21,6 +21,18 @@ radio {
 .osdheaderbar button {
   border: transparent;
 }
+/* Non-osd style for popover menu */
+.menupopover label {
+  color: @theme_text_color;
+}
+.menupopover arrow {
+  background: @theme_base_color;
+  border-color: @insensitive_base_color;
+}
+.menupopover contents {
+  background: @theme_base_color;
+  border-color: @insensitive_base_color;
+}
 .adwrounded.csd {
   border-radius: 8px;
 }