Expose default theme in meta and API (#13809)

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

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

routers/api/v1/settings/settings.go

@@ -23,6 +23,7 @@ func GetGeneralUISettings(ctx *context.APIContext) {
 	//   "200":
 	//     "$ref": "#/responses/GeneralUISettings"
 	ctx.JSON(http.StatusOK, api.GeneralUISettings{
+		DefaultTheme:     setting.UI.DefaultTheme,
 		AllowedReactions: setting.UI.Reactions,
 	})
 }