@leocg, There is many examples. Apparently, it occours in pages different form main page, even if login is not required. There is a problem even with opera sites:
Note that it's different from 'Dark mode'. This issue is really forcing a dark background without clear fonts. Dark mode makes the page become more clear.
I also have this problem sometimes when attepting to print a gmail message: That 'clear view' for print also becomes black backgrounded with black text.
Sometimes is all ok with the same sites that usually show the problem.
This post is a dupplicate from this one: https://forums.opera.com/topic/43798/dark-theme-in-pages-is-forced-even-when-not-selected/67?lang=es
No solution until now!