Fix selection of parameter names in HTML theme

Parameters are written in HTML as (leaving out some internal classes):
```
<strong>name</strong><span class="classifier"><a><code><span>TypeName</span></code></a></span>
```
but in rendered form there's a colon between the name and type. This
colon is inserted virtually using CSS, but since it doesn't exist, the
browser thinks both sides are part of the same word.

Styling the virtual text as inline block makes it be treated as a break,
but also makes it apply vertical margins, so we need to set those to
zero again.

See matplotlib/matplotlib#21432

sphinx/themes/basic/static/basic.css_t

@@ -731,8 +731,9 @@ dl.glossary dt {
 
 .classifier:before {
     font-style: normal;
-    margin: 0.5em;
+    margin: 0 0.5em;
     content: ":";
+    display: inline-block;
 }
 
 abbr, acronym {