[guuhv]

It's the default on Linux, "use system colours" is on by default, and the default background colour for widgets on Linux is usually not white.

[deadbunny]

You mean the default in whatever flavour of linux you're using. It's not enabled by default on all distros.

[hannasanarion]

It's the default in Firefox for Linux. It's a browser setting, not an OS setting. And in order to turn it off you have to do really obscure stuff in the Firefox launch options, it's funky and makes no sense.