Dark mode backport to v11

[aMediocreDad]

Describe the bug

Dark mode is not working in v11, because foundry v11 does not have a theme setting.

Reproduction

NA

System Info

NA

Logs

No response

Validations

• Done a clean install of the system.
• Disabled all modules.
• Checked if the issue is present in a new world instance (create a new world in foundry using the Forbidden Lands system).
• Not a bug report about the content modules. Reports regarding the content modules go here instead.
• Not a "How To"-question (You can ask questions here: Discussions).