Expose default theme in meta and API (#13809)

Signed-off-by: jolheiser <john.olheiser@gmail.com>

Co-authored-by: 6543 <6543@obermui.de>

modules/structs/settings.go

@@ -12,6 +12,7 @@ type GeneralRepoSettings struct {
 
 // GeneralUISettings contains global ui settings exposed by API
 type GeneralUISettings struct {
+	DefaultTheme     string   `json:"default_theme"`
 	AllowedReactions []string `json:"allowed_reactions"`
 }