Modify Doxygen navbar to link to Sphinx docs

doc/doxygen/Doxyfile

@@ -1213,7 +1213,8 @@ HTML_STYLESHEET        =
 # list). For an example see the documentation.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_EXTRA_STYLESHEET  = ext/doxygen-awesome-css/doxygen-awesome.css
+HTML_EXTRA_STYLESHEET  = ext/doxygen-awesome-css/doxygen-awesome.css \
+                         doc/doxygen/cantera-doxygen.css
 
 # The HTML_EXTRA_FILES tag can be used to specify one or more extra images or
 # other source files which should be copied to the HTML output directory. Note

doc/doxygen/cantera-doxygen.css

@@ -0,0 +1,38 @@
+/* Colors to match pydata-sphinx-theme */
+html {
+    --primary-color: rgb(69, 157, 185);
+    --pst-color-text-muted: rgb(100, 100, 100);
+}
+
+html.dark-mode {
+    --primary-color: rgb(69, 157, 185);
+    --pst-color-text-muted: rgb(164, 164, 164);
+}
+
+ul#main-menu li a {
+    /* Hide Doxygen navigation tabs that are duplicated in the sidebar */
+    visibility: hidden;
+    display: none !important;
+}
+
+div#top {
+    /* Fix alignment of nav tabs added using header.html */
+    justify-content: left;
+}
+
+/* Re-style the nav bar links to approximate pydata-sphinx-theme */
+.sm-dox a {
+    color: var(--pst-color-text-muted) !important;
+    font-size: 16px;
+}
+
+.sm-dox a#selected {
+    color: var(--primary-color) !important;
+    font-weight: bold;
+}
+
+.sm-dox a:hover {
+    background: unset;
+    color: var(--primary-color) !important;
+    font-size: 16px;
+}

doc/doxygen/header.html

@@ -69,5 +69,11 @@ $extrastylesheet
  </tbody>
 </table>
 </div>
+<div>
+  <ul class="sm sm-dox" style="display: block;">
+    <li><a id="selected" href="$relpath^../../sphinx/html/reference.html">Reference</a></li>
+    <li><a href="$relpath^../../sphinx/html/examples.html">Examples</a></li>
+  </ul>
+</div>
 <!--END TITLEAREA-->
 <!-- end header part -->