It seems to be platform-wide, but the dark mode seems well... Only on the top of the page and until a certain measure. At 100% zoom on HD screen (=1080p), the part of the left menu and of the bottom menus is on a white background. Unzoom and the black covers a bit more. Only a 50% zoom seems to "hide" the issue.
It's like if now the dark mode was using a fixed size black background that stopped at a certain moment. I think this probably started this month since last month the dark mode was working correctly. Content part is NOT affected. It's only the menus.
Right now, let's say, if I go on Meta. While being on Firefox, at 100% zoom the dark mode stops at the "Tools" menu. All that is below "Tools" menu ("Tools" menu included) and that is not in the main content part is on a white background.
Tested on Firefox and Chrome, latest versions. This doesn't seem to be specific to a web browser. Under Vector skin (default). Didn't test other skins yet.