Update css to support diff syntax highlighting. (#943)

This adds the rules to highlight diff lines with highlight.js.

src/theme/ayu-highlight.css

@@ -69,3 +69,11 @@ Original by Dempfi (https://github.com/dempfi/ayu)
 .hljs-strong {
   font-weight: bold;
 }
+
+.hljs-addition {
+  color: #91b362;
+}
+
+.hljs-deletion {
+  color: #d96c75;
+}

src/theme/highlight.css

@@ -67,3 +67,13 @@
 .hljs-strong {
   font-weight: bold;
 }
+
+.hljs-addition {
+  color: #22863a;
+  background-color: #f0fff4;
+}
+
+.hljs-deletion {
+  color: #b31d28;
+  background-color: #ffeef0;
+}

src/theme/tomorrow-night.css

@@ -94,3 +94,11 @@
 .xml .hljs-cdata {
   opacity: 0.5;
 }
+
+.hljs-addition {
+  color: #718c00;
+}
+
+.hljs-deletion {
+  color: #c82829;
+}