MDL-40975 Theme: Set the colour for actionmenu carets appropriately

theme/base/style/core.css

@@ -1595,8 +1595,13 @@ a.disabled {
     width: 0;
     height: 0;
     vertical-align: top;
-    border-top: 4px solid #000;
+    border-top: 4px solid #777;
     border-right: 4px solid transparent;
     border-left: 4px solid transparent;
     content: "";
 }
+
+a:focus .caret,
+a:hover .caret {
+    border-top-color: #555;
+}

theme/bootstrapbase/less/moodle/core.less

@@ -2045,6 +2045,11 @@ img#persona_signin {
             .caret {
                 margin-top: 8px;
                 margin-left: 2px;
+                border-top-color: @navbarLinkColor;
+                &:hover,
+                &:active {
+                    border-top-color: @navbarLinkColorActive;
+                }
             }
         }
     }