DOC: Highlight literals a bit more

The only style difference for literals is currently a monospace font.
This blends very much into surrounding text and makes it hard to read
the literals.

All other major themes (https://sphinx-themes.org/) use a background
for literals. It's a good idea to do the same in sphinx13. I've taken the
CSS from pydata-sphinx-theme, but I don't care too much about the actual
parameters as long as it stays visible but decent.
Note that I've limitied the background to `div.body` so that the background
is not applied in the sidebar. The sidebar entries are often single words
phrases and don't need a background. Actually, the backgound there would
be too cluttery.

doc/_themes/sphinx13/static/sphinx13.css

@@ -321,6 +321,13 @@ cite, code, tt {
     letter-spacing: -0.02em;
 }
 
+div.body code.literal {
+    background-color: #f3f4f5;
+    border: 1px solid #d1d5da;
+    border-radius: 0.25rem;
+    padding: .1rem .2rem;
+}
+
 table.deprecated code.literal {
     word-break: break-all;
 }