I have been having the exact same problem. I have Enabled and Disabled the built-in System Theme that is suppose to be dark at top but everything remains Very White / Grey. And no updates are available when checked. Please help!

I also tried to change between System and Light, but there is no difference. I am finding it too bright to use. Dark is too black on the main area of the screen. I want to revert to the previous version I had until Firefox updated this morning. Please help as I am finding it too difficult to use Firefox.

I can't believe Mozilla Firefox would make suck a mistake and make light theme and System theme exactly the same thing, this must be a BUG and please FIX this Firefox as I don't like it so white and light, bring back the blue! Otherwise you will start to lose followers for sure!

I think someone has filed a bug but to be honest, it is hard to keep track of all the threads in recent days. Ahem.

Meanwhile, if you prefer the colors of the "Default" theme from Firefox 57-88, someone created an add-on theme that emulates it. You can give it a try here:

https://addons.mozilla.org/firefox/addon/photon-colors/

If that color doesn't suit you, the Firefox Color extension lets you create your own theme:

https://addons.mozilla.org/firefox/addon/firefox-color/