FIX Use SSViewer.theme_enabled in CMS logic

The previously used SSViewer::current_custom_theme() has been
removed since we can't influence the setting of configuration values.

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.

Depends on https://github.com/silverstripe/sapphire/pull/1340