Fix menu bar color.

Since the font color is set generally for widgets
the menu bar did not get its individual color.
The css rule got  refined to be not overwritten by the p-widget rule.

Signed-off-by: Jan Bicker <jan.bicker@typefox.io>

packages/core/src/browser/style/menus.css

@@ -30,7 +30,7 @@
 |----------------------------------------------------------------------------*/
 
 
-.p-MenuBar {
+.p-Widget.p-MenuBar {
   padding-left: 5px;
   background: var(--theia-menu-color0);
   color: var(--theia-ui-bar-font-color1);