Standardize tooltip colors

Also removes the mixin for customizing tooltip background, as the only
use of this mixin was for a color that is almost indistinguishable from
the default one.

Change-Id: Ic1be80a78d586e5ebed04893bc8b715f7d4a47a3

polygerrit-ui/app/elements/diff/gr-selection-action-box/gr-selection-action-box.html

@@ -29,9 +29,6 @@ limitations under the License.
         position: absolute;
         white-space: nowrap;
       }
-      #tooltip {
-        --tooltip-background-color: rgba(22, 22, 22, .9);
-      }
     </style>
     <gr-tooltip
         id="tooltip"

polygerrit-ui/app/elements/shared/gr-alert/gr-alert.html

@@ -29,7 +29,7 @@ limitations under the License.
        * HOW THEY ARE USED IN THE CODE.
        */
       :host([toast]) {
-        background-color: #333;
+        background-color: var(--tooltip-background-color);
         bottom: 1.25rem;
         border-radius: 3px;
         box-shadow: 0 1px 3px rgba(0, 0, 0, .3);
@@ -45,6 +45,7 @@ limitations under the License.
         transform: translateY(0);
       }
       .text {
+        color: var(--tooltip-text-color);
         display: inline-block;
         max-height: 10rem;
         max-width: 80vw;
@@ -52,7 +53,7 @@ limitations under the License.
         word-break: break-all;
       }
       .action {
-        color: #a1c2fa;
+        color: var(--link-color);
         font-family: var(--font-family-bold);
         margin-left: 1em;
         text-decoration: none;

polygerrit-ui/app/elements/shared/gr-tooltip/gr-tooltip.html

@@ -25,9 +25,9 @@ limitations under the License.
         --gr-tooltip-arrow-size: .5em;
         --gr-tooltip-arrow-center-offset: 0;
 
-        background-color: var(--tooltip-background-color, #333);
+        background-color: var(--tooltip-background-color);
         box-shadow: 0 1px 3px rgba(0, 0, 0, .3);
-        color: #fff;
+        color: var(--tooltip-text-color);
         font-size: var(--font-size-small);
         position: absolute;
         z-index: 1000;
@@ -53,11 +53,11 @@ limitations under the License.
         width: 0;
       }
       .arrowPositionAbove {
-        border-top: var(--gr-tooltip-arrow-size) solid var(--tooltip-background-color, #333);
+        border-top: var(--gr-tooltip-arrow-size) solid var(--tooltip-background-color);
         bottom: -var(--gr-tooltip-arrow-size);
       }
       .arrowPositionBelow {
-        border-bottom: var(--gr-tooltip-arrow-size) solid var(--tooltip-background-color, #333);
+        border-bottom: var(--gr-tooltip-arrow-size) solid var(--tooltip-background-color);
         top: -var(--gr-tooltip-arrow-size);
       }
     </style>

polygerrit-ui/app/styles/app-theme.html

@@ -87,6 +87,9 @@ limitations under the License.
 
   --edit-mode-background-color: #ebf5fb;
 
+  --tooltip-background-color: #333;
+  --tooltip-text-color: #fff;
+
   --syntax-default-color: var(--primary-text-color);
   --syntax-meta-color: #FF1717;
   --syntax-keyword-color: #9E0069;