Add some spacing below h3 headers in HTML docs

docs/source/_static/default.css

@@ -277,7 +277,7 h2 {
 }
 
 h3 {
-    margin: 1em 0 -0.3em 0;
+    margin: 1em 0 0.2em 0;
     font-size: 1.2em;
 }
 
@@ -531,4 +531,4 span.versionmodified {
 
 pre {
   white-space: pre-wrap;
-}
  No newline at end of file
+}