Refactor syntax highlighting palette

Puts the palette into variables set by the app theme.

TODO: Refactor the app theme into app theme and syntax theme, but keep
the variable usage added in this change.

Change-Id: I4fe0789af3f7bbc1b8b0568f62bc58814a0d571d
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 f45b2d3..c7b89c1 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -25,7 +25,7 @@
 <link rel="import" href="../gr-diff-comment-thread/gr-diff-comment-thread.html">
 <link rel="import" href="../gr-diff-highlight/gr-diff-highlight.html">
 <link rel="import" href="../gr-diff-selection/gr-diff-selection.html">
-<link rel="import" href="../gr-syntax-themes/gr-theme-default.html">
+<link rel="import" href="../gr-syntax-themes/gr-syntax-theme.html">
 
 <script src="../../../scripts/hiddenscroll.js"></script>
 
@@ -257,7 +257,7 @@
         background-repeat: repeat-y;
       }
     </style>
-    <style include="gr-theme-default"></style>
+    <style include="gr-syntax-theme"></style>
     <div id="diffHeader" hidden$="[[_computeDiffHeaderHidden(_diffHeaderItems)]]">
       <template
           is="dom-repeat"