fix code highlight: add cppmode theme and mode to reference/error/examples

Author: pepc84

reference/index.html

@@ -23,6 +23,8 @@ <h1 style="font-family:inherit;font-size:2rem;margin-bottom:0.5rem;">Reference</
 <script src="https://cdnjs.cloudflare.com/ajax/libs/codemirror/5.65.16/codemirror.min.js"></script>
 <script src="../assets/cppmode-keywords.js"></script>
 <script src="../assets/cm-cppmode.js"></script>
+<script src="../assets/cppmode-keywords.js"></script>
+<script src="../assets/cm-cppmode.js"></script>
 <script src="../assets/code-highlight.js"></script>
 <script src="../assets/nav.js"></script>
 </body>