Update styles for shadow dom

- Create a shared style module that is included in every custom element
- Add the shared style module to each existing element

Change-Id: I1ee382955afe4ff630548a6640e7c4d03688849d
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 056c02e..687c462 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -22,12 +22,13 @@
 <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="../../../styles/shared-styles.html">
 
 <script src="../../../scripts/hiddenscroll.js"></script>
 
 <dom-module id="gr-diff">
   <template>
-    <style>
+    <style include="shared-styles">
       :host {
         --light-remove-highlight-color: #fee;
         --dark-remove-highlight-color: rgba(255, 0, 0, 0.15);