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.
Accessing Loaded Themes
Get an array of all the loaded theme names.
Get an array of all the loaded themes.
Accessing Active Themes
Get an array of all the active theme names.
Get an array of all the active themes.
Managing Enabled Themes
Get the enabled theme names from the config.
Returns an array of theme names in the order that they should be activated.