Add automatic light/dark option for the colorblind theme (#31997)

The normal themes already have a variant which automatically chooses
light/dark mode based on the browser.
This PR adds the same variant, but for the colorblind themes.

web_src/css/themes/theme-gitea-auto-protanopia-deuteranopia.css

@@ -0,0 +1,2 @@
+@import "./theme-gitea-light-protanopia-deuteranopia.css" (prefers-color-scheme: light);
+@import "./theme-gitea-dark-protanopia-deuteranopia.css" (prefers-color-scheme: dark);