Add support for the new Github UI Overhaul

[Lunchb0ne]

Describe the bug: The theme is currently broken on the new Github UI overhaul.

Expected behavior: The theme should work fine.

Where did this happen: The repo/link at which the issue occurred.

Steps to reproduce: Go to feature preview and enable the new UI preview.

Screenshots: Buttons like these are not properly themed

Desktop (please complete the following information):

• OS: Windows 10
• Browser: Chrome

• Version: Newest

  Additional add-ons installed: A list of extensions you are using besides Stylus. LoverlyForks Octolinker Gitako HideFiles

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