🎨 remove all string background in dark mode (#354)

## Description

This PR further updates the CSS configuration for the documentation to
nicely render strings in dark mode.

## Checklist:

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- [x] The pull request only contains commits that are related to it.
- [x] I have added appropriate tests and documentation.
- [x] I have made sure that all CI jobs on GitHub pass.
- [x] The pull request introduces no new warnings and follows the
project's style guidelines.

Signed-off-by: burgholzer <burgholzer@me.com>

docs/source/_static/custom.css

@@ -22,9 +22,19 @@
 }
 
 /* override the default background color for literal strings */
-body:not([data-theme="light"]) .highlight .s1 {
-  background-color: #00000033;
-}
-body:not([data-theme="light"]) .highlight .s2 {
-  background-color: #00000033;
+body:not([data-theme="light"]) .highlight .sa,
+.highlight .sb,
+.highlight .sc,
+.highlight .dl,
+.highlight .sd,
+.highlight .s2,
+.highlight .se,
+.highlight .sh,
+.highlight .si,
+.highlight .sx,
+.highlight .sr,
+.highlight .s1,
+.highlight .ss,
+.highlight .s1 {
+  background-color: #00000001;
 }