Introduce deemphasized-text-color

Introduces and standardizes the various shades of medium dark gray used
throughout the app to #616161, or Material Gray 600.

Change-Id: I311155cfdba4381643451233dfafbfbdacd55e90
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
index dfd57e4..3414266 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -110,7 +110,7 @@
         -ms-user-select: none;
         user-select: none;
 
-        color: #666;
+        color: var(--deemphasized-text-color);
         padding: 0 .5em;
         text-align: right;
       }
@@ -164,7 +164,7 @@
       .contextControl gr-button {
         display: inline-block;
         text-decoration: none;
-        --gr-button-color: rgba(0,0,0,.54);
+        --gr-button-color: var(--deemphasized-text-color);
         --gr-button: {
           padding: .2em;
         }