themes: Define merged titlebar icon colors

Resolves #11552
We did not explicitly define the colors for the Minimize,
Maximize, and Close button icons for merged titlebars.
On some system themes, the color used blends into
the titlebar and makes the icon look invisible.
This patch makes sure we always define the icon color
for the Default theme.

themes/Default/common.css

@@ -817,6 +817,11 @@ headerbar button.titlebutton, headerbar > menubar {
   box-shadow: none;
 }
 
+/* Define icon colors for merged headerbar buttons */
+.minimize image, .maximize image, .close image {
+  color: @scrollbar-slider-color;
+}
+
 .titlebar {
   padding-top: 0px;
   padding-bottom: 0px;