Define table-subheader-background-color

Defines a CSS constant and uses it throughout the app.

Change-Id: I610e8eb4a51aa72e8eea526c585f308cabb55169

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

@@ -72,7 +72,7 @@ limitations under the License.
       /* Keep below color definition for primary/secondary so that this takes
        precedence when disabled. */
       :host([disabled]) {
-        --background-color: #eaeaea;
+        --background-color: var(--table-subheader-background-color);
         --button-color: #a8a8a8;
         cursor: default;
       }

polygerrit-ui/app/elements/shared/gr-page-nav/gr-page-nav.html

@@ -22,7 +22,7 @@ limitations under the License.
   <template>
     <style include="shared-styles">
       #nav {
-        background-color: #f5f5f5;
+        background-color: var(--table-header-background-color);
         border: 1px solid var(--border-color);
         border-top: none;
         height: 100%;

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

@@ -36,6 +36,7 @@ limitations under the License.
     transition: none;
   }
   --table-header-background-color: #fafafa;
+  --table-subheader-background-color: #eaeaea;
 
   /* Font sizes */
   --font-size-normal: 1rem;

polygerrit-ui/app/styles/gr-change-list-styles.html

@@ -80,7 +80,7 @@ limitations under the License.
         top: 0;
       }
       .groupHeader {
-        background-color: #eaeaea;
+        background-color: var(--table-subheader-background-color);
       }
       .groupHeader a {
         color: var(--primary-text-color);