Make primary disabled button look more disabled

Bug: Issue 4300
Change-Id: I7d997d3a33c9e86e0dfcf32a345fb31717e19c6a
diff --git a/polygerrit-ui/app/elements/shared/gr-button/gr-button.html b/polygerrit-ui/app/elements/shared/gr-button/gr-button.html
index cc0da66..164bb2d 100644
--- a/polygerrit-ui/app/elements/shared/gr-button/gr-button.html
+++ b/polygerrit-ui/app/elements/shared/gr-button/gr-button.html
@@ -120,13 +120,14 @@
       :host([secondary]:active) {
         border-color: #941c0c;
       }
-      :host([primary][loading]),
-      :host([primary][disabled]) {
+      :host([primary][loading]) {
         background-color: #7caeff;
         border-color: transparent;
         color: #fff;
       }
-
+      :host([primary][disabled]) {
+        background-color: #888;
+      }
     </style>
     <content></content>
   </template>