I run a node server (server.js), then I can see my website at http://localhost:3000

using edge as browser all colors of the website are as I would expect. Using Opera they are all darker or i.e. black text got turned in to white. So it sounds like Opera is forcing a dark mode.
It wasn't allways like this. I even think I misclicked something when I wanted to cklick on "analyse element". I think it was then that it started showing the wrong colors. But I don't know what I clicked and also not how do undo it.

I have the theme set to dark for Opera and I would like to keep it like this. I don't have "force dark site" enabled. Chaining the theme to light did have an effect either.
Also disabeling "Enable Force dark theme on pages from Settings and Easy Setup" and "Auto Dark Mode for Web Contents" under "opera://flags" had no effect either.

Also I don't knwo if that is related but sometimes it still used some old chached version of my html, css, etc files even after reload. I this cases I used the incogni winodw. But for the dark mode problem even restarting my pc did not help.

Any cule what is going on?