Use --deemphasized-text-color in context expander

A recent change removed the --gr-button-color mixin, but did not set the
proper button color in the context expander.

Change-Id: I92393097fb4c3cc8b7b024ea90bac9d63610e81d

polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html

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 47b6e60..7c7ce06 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -155,6 +155,7 @@
         display: inline-block;
         text-decoration: none;
         --gr-button: {
+          color: var(--deemphasized-text-color);
           padding: .2em;
         }
       }