Handles loading and activating available themes.
An instance of this class is always available as the atom.themes global.
Invoke callback when style sheet changes associated with
updating the list of active themes have completed.
| Argument | Description | 
|---|---|
| 
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| Return values | 
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| Returns a Disposable on which  | 
Get the enabled theme names from the config.
| Return values | 
|---|
| Returns an array of theme names in the order that they should be activated. |