This will ultimately end up on Github, I'm just temporarily storing this here. It still has a few things here and there I need to fix. ProtonMail allows you to save the styles within PM Settings, so that would be the preferred method of using this theme to redesign PM.

This is currently saved WITHOUT the background image in the screenshots as base64 (which came to 56kb), but with it as an imgur link. Obviously not ideal if using ProtonMail. But good for testing.