Well, the geniuses at Firefox have done it again with version 89: Dumb down the format, make it bland so that tabs and icons don't stand out as they did before. Why? Because they need something to justify their jobs. So, it's this theme which restores the vibrancy of the display ... until the next time the geniuses want to make a change.

EDIT: I have been browsing with this theme enabled for over an hour now, and guess what? You know what they say about being handed lemons and making lemonade? I actually like this theme BETTER than the default theme that was replaced by version 89. If I had known about this theme before, I would have adopted it already but I was not incentivized to look until version 89 came along. Good job, thank you.