Use light/dark theme based on system preference (#17051)

Add a new default theme `auto`, which will automatically switch between
`gitea` (light) and `arc-green` (dark) themes depending on the user's
operating system settings.

Closes: #8183

web_src/less/themes/theme-auto.less

@@ -0,0 +1,3 @@
+@media (prefers-color-scheme: dark) {
+  @import "theme-arc-green.less";
+}