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/dtos/index.go

@@ -62,3 +62,6 @@ type NavLink struct {
 	HideFromTabs bool       `json:"hideFromTabs,omitempty"`
 	Children     []*NavLink `json:"children,omitempty"`
 }
+
+// NavIDCfg is the id for org configuration navigation node
+const NavIDCfg = "cfg"