@DeltaKestrel Hmm, I can't get it to happen on chatgpt.com no matter how I config things. Does it happen in a test standalone installation too?
For your regular installs, I'd goto the URL opera://settings/content/all, type chatgpt.com in the search field and click "Delete displayed data". Then, repeat for openai.com. Then, goto the URL opera://settings/clearBrowserData and clear "Cached images and files" for all time. Then, log back in to chatgpt.com and see if the issue still happens.
Note also that sometimes force dark mode can get stuck on in some rare cases where the only way to fix it is to goto the URL opera://settings/reset and reset just your settings. You'll usually lose open tabs though. Trying in a test standalone installation will tell you if resetting your settings will even help.
Are all 3 computers on Windows?
Have you messed with the "Forced colors" and "Force color profile" settings at the URL opera://flags?
Is there any difference if you switch to another them and switch back to the Classic theme? Maybe reset the classic themes settings too if you haven't.
Any difference with "graphics acceleration" disable at the URL opera://settings/system?