Add Ability to Hide Sun/Moon Toggle

[spaceninja]

By default the Storybook UI will switch between the light and dark theme, but once you customize the theme, that feature goes away.

I'm using this plugin to provide a custom light and dark theme and preserve the ability for them to shift automatically with the system.

However, I don't actually want the sun/moon toggle. Is there a way to suppress it, while keeping the functionality?

[hipstersmoothie]

Not currently. Would merge a PR that adds this thought!