The forum has not changed the theme till now. You activate the dark mode on your browser is the reason. Many browsers on phones and computers are now like this.
On my Windows computer Edge browser, the dark mode comes with it by default on my computer.
You can change that on your browser settings.
Many browsers on mobile especially on android don't have dark mode and for some the dark mode is pretty messed up. Chrome for example is a very common and popular browser on mobile and it doesn't have a very responsive dark mode. Fun thing is I for some reason just prefer this forum in light theme.
Infact I think this is the longest place I surf on light mode because I use dark mode most of the time outside the forum, well I guess photochromic glasses cover for that.