Theming: Support for runtime theme switching and hooks for custom themes (#31301)

* WIP Custom themes

* Load custom themes from URL and via event

* Dynamic page background

* Header color change

* Fixing tests and emotion warnings

* Fixed test

* moving cx to getStyles

* Review fixes

* minor change

pkg/api/index.go

@@ -282,7 +282,7 @@ func (hs *HTTPServer) getNavTree(c *models.ReqContext, hasEditPerm bool) ([]*dto
 
 	if len(configNodes) > 0 {
 		navTree = append(navTree, &dtos.NavLink{
-			Id:         "cfg",
+			Id:         dtos.NavIDCfg,
 			Text:       "Configuration",
 			SubTitle:   "Organization: " + c.OrgName,
 			Icon:       "cog",