You don't find themes like Simple White¹ or FT DeepDark² to be acceptable?

1: https://addons.mozilla.org/en-US/firefox/addon/simplewhite/

2: https://addons.mozilla.org/en-US/firefox/addon/ft-deepdark/

The themes don't change the arrangement of the UI.

Try Debian if you need something so rock solid that you can only get security updates.

I do use Debian for my desktop. So? I still have Australis. The themes don't change that fact.

If you're on Windows or Linux try Palemoon [0].

[0]: http://www.palemoon.org/