docs: fix double-clicking on member names

In the rendered html docs, double-clicking a name, such as a class name,
would select both the keyword "class" as well as the class name. This
happens because there is no whitespace between the two HTML elements.
We can patch this by injecting a space using CSS.

Issue: readthedocs/sphinx_rtd_theme#1154

docs/_static/css/custom.css

@@ -1,3 +1,9 @@
+/* Fix coloring to be like older versions of Sphinx + RTD theme. */
 .descname {
     color: black;
 }
+
+/* Fix double-click to select single word. */
+html.writer-html5 .rst-content dl[class]:not(.option-list):not(.field-list):not(.footnote):not(.glossary):not(.simple) .property::after {
+    content: " ";
+}