Fixed toolbar buttons style, removing unneeded outline

BUG: 443626


(cherry picked from commit 23ba2a8d)

src/gtk3/widgets/_button.scss

@@ -176,6 +176,7 @@ $_dot_color: gtk("@theme_button_decoration_focus");
 
   @include button(normal);
   &.flat {
+    @include button(undecorated);
     // to avoid adiacent buttons borders clashing when transitioning, the transition on the normal state is set
     // to none, while it's added back in the hover state, so the button decoration will fade in on hover, but
     // it won't fade out when the pointer leave the button allocation area. To make the transition more evident
@@ -228,12 +229,15 @@ $_dot_color: gtk("@theme_button_decoration_focus");
   }
   &.flat:backdrop {
     -gtk-icon-effect: none;
+    @include button(undecorated);
     color: gtk("@theme_button_foreground_backdrop");
   }
   &.flat:disabled {
+    @include button(undecorated);
     color: gtk("@theme_button_foreground_insensitive");
   }
   &.flat:backdrop:disabled {
+    @include button(undecorated);
     color: gtk("@theme_button_foreground_backdrop_insensitive");
   }
   &:disabled {
@@ -357,6 +361,7 @@ button {
       &.flat:backdrop,
       &.flat:disabled,
       &.flat:backdrop:disabled {
+        @include button(undecorated);
         color: gtkalpha($button_color, 0.8);
       }
       &:disabled {