gui, lib/api: Adds support for prefers-color-scheme (fixes #6115)

* gui, lib/api: Adds support for prefers-color-scheme on default theme (fixes #6115)

- Renames current default theme into a new "light" theme
- Modifies assets serving to allow getting assets from different themes

* lib/api: Serve assets from arbitrary theme when path starts with "theme-assets"

* lib/api: Moves constant out of function

* Loads light theme in browsers without support for prefers-color-scheme

* gui: Disables dark theme when printing

* Prevents repeated injection and adds support for older browsers

The CSS is always loaded if there is no support for `matchMedia`.

lib/api/api_statics.go

@@ -23,6 +23,8 @@ import (
 	"github.com/syncthing/syncthing/lib/sync"
 )
 
+const themePrefix = "theme-assets/"
+
 type staticsServer struct {
 	assetDir        string
 	assets          map[string][]byte
@@ -91,6 +93,20 @@ func (s *staticsServer) serveAsset(w http.ResponseWriter, r *http.Request) {
 	modificationTime := s.lastThemeChange
 	s.mut.RUnlock()
 
+	// If path starts with special prefix, get theme and file from path
+	if strings.HasPrefix(file, themePrefix) {
+		path := file[len(themePrefix):]
+		i := strings.IndexRune(path, '/')
+
+		if i == -1 {
+			http.NotFound(w, r)
+			return
+		}
+
+		theme = path[:i]
+		file = path[i+1:]
+	}
+
 	// Check for an override for the current theme.
 	if s.assetDir != "" {
 		p := filepath.Join(s.assetDir, theme, filepath.FromSlash(file))