Disable light/dark toggle button

[TDuffinNTU]

Is your feature request related to a problem? Please describe.
There isn't any way to disable the light/dark theme toggle.

Describe the solution you'd like
I'd like to have a single theme, instead of "light vs dark". Though this still keeps the toggle, which is now useless.

Describe alternatives you've considered
Right now, I've just commented out the light theme in my config.yaml

Additional context
A theme-toggle tag that can be given false/light/dark to disable/set a default theme could be cool?