Always use the body background for notification menu

On small screen the menu inherits of the navbar background which reduces the
contrast with the item of the menu.

Closes #14855

sao/src/sao.less

@@ -441,6 +441,9 @@ html[theme="default"] {
     }
 
     ul.notification-menu {
+        background-color: @body-bg;
+        color: @text-color;
+
         @media (min-width: @grid-float-breakpoint) {
             width: 320px;
         }
@@ -453,6 +456,7 @@ html[theme="default"] {
             }
 
             > a {
+                color: @text-color;
                 display: flex;
                 padding: 3px 10px;
 
@@ -489,6 +493,10 @@ html[theme="default"] {
                 }
             }
         }
+
+        > li.divider {
+            background-color: @dropdown-divider-bg;
+        }
     }
 }