doc: import Sphinx theme.css and pygments.css

We want to override the Sphinx theme only partially. So we need to
import the Sphinx theme.css. Cf.
https://docs.readthedocs.io/en/stable/guides/adding-custom-css.html

We also need to import the pygments.css for syntax highlighing.

Use sans-serif for body.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/sphinx-static/theme_overrides.css

@@ -5,10 +5,13 @@
  * work for both normal and dark modes.
  */
 
+@import 'css/theme.css';
+@import 'pygments.css';
+
 /* Improve contrast and increase size for easier reading. */
 
 body {
-	font-family: serif;
+	font-family: sans-serif;
 	font-size: 100%;
 }