Edit: Some sites have elements hidden while in Dark Mode. This has given me problems. I can edit the theme for such sites BUT - a suggestion: when editing/creating a theme, please make the default to only do so for the CURRENT SITE. "All sites" should be explicitly chosen by the user. Thank you!

This is a great add-on and I keep it ON by default. One important improvement for me would be this:
The default behavior for text fields (e.g., username/password fields) gives me yellow background with white lettering. It would be better to have either darker lettering or darker background. As it is, it is very hard (next to impossibile) to make out anything.
Other than that, perfect! THANK YOU :-)