API Replaced SSViewer.custom_theme with SSViewer.theme_enabled

Since we can't influence the setting of configuration values,
we also can't set/unset the 'custom_theme' value based on which
theme is set. This means the 'custom_theme' value goes stale,
and we can't rely on it e.g. in FilesystemPublisher.

The 'theme_enabled' toggle is a cleaner solution to the same problem,
since the 'custom_theme' was really just a way to remember the original
theme, while still disabling it. The toggle makes this more explicit,
but also requires users of the 'theme' setting to check for it.