[BRANDING] Add Forgejo light, dark, and auto themes: fix import

Closes: https://codeberg.org/forgejo/forgejo/issues/562
(cherry picked from commit 2b0dc1f80ff18d1c94df74c77a80e3651e7cb2ed)
(cherry picked from commit 690dde103fa444efb5087c1565d27ef1e3e4607e)
(cherry picked from commit 1443ea4620f1eba80faa661e85025b2f2f820e69)
(cherry picked from commit 39de9bda2af8338b291a797ae74377819e167497)
(cherry picked from commit 0c20477a75a47a17fd9556cf4f9f21bc473eaeb0)

Author: earl-warren

web_src/css/themes/theme-forgejo-auto.css

@@ -1,4 +1,2 @@
 @import "theme-forgejo-light.css";
-@media (prefers-color-scheme: dark) {
-  @import "theme-forgejo-dark.css";
-}
+@import "theme-forgejo-dark.css" (prefers-color-scheme: dark);