Bad background color in my Firefox Nightly, can't figure this one

[gorhill]

For the longest time I noticed the default background color in my Firefox Nightly is darker than that of default profiles or that of Firefox stable.

Today I wanted to look more into why, and I find this:

So there is this bad CSS color somewhere missing a leading #. I have no idea where is that CSS declaration, I can't find a such base.css file anywhere -- and I can't find any userChrome.css or userContent.css anywhere in my home older.

Any of you know how to get to the bottom of this?

[gwarser]

https://searchfox.org/mozilla-central/rev/6b0b8c1003504e302e243c300355ef4d61e24909/devtools/client/aboutdebugging/src/base.css#7

[gorhill]

Ok, I see the same faulty CSS color in a new profile, but the background is white (or close to it) in that new profile. I get that same dark gray background everywhere no background color is declared (including webpages), or when I uncheck those CSS declarations. I don't get this dark gray in new profile or Firefox stable. I have no idea where that dark gray is coming from.

This is what made me look into this this morning: https://dada-lang.org/docs/dyn_tutorial/. It's very difficult to read with that dark gray background:

Any other profile, either Firefox or Chromium is fine.

[gorhill]

Ok, found the cause, it's browser.display.use_system_colors in about:config, it was true (not default). Toggling back false fixes it. I typically never change about:config stuff, so I might have changed a long time ago for debugging purpose (maybe related to dark theme work) and forgot to reset it.