Handles loading and activating available themes.
An instance of this class is always available as the
callback when style sheet changes associated with
updating the list of active themes have completed.
Get an array of all the loaded theme names.
Get an array of all the loaded themes.
Get an array of all the active theme names.
Get an array of all the active themes.
Get the enabled theme names from the config.
Returns an array of theme names in the order that they should be activated.