This is not the behaviour observed in Chrome

I think Opera does its behavior on purpose where it has the additional force dark pages control that the flag depends and only-dark-theme behavior.

Have you ever refreshed a website using dark mode and seen short-lived white flashes

Yeah, most users call these flash bangs. There are lots of situations that these can happen. See https://issues.chromium.org/issues/40581308 for an example in Chrome. They're not always easy to fix. You're also not the only user to complain about them in Opera.

Then I tried installing Brave browser, and it turns out everything works just as expected in it when #enable-force-dark is enabled.

I think someone needs to look at opera's code and get it fixed at some point. I'm not coder, I wish I could help. So I'll leave it in the capable hands of the custodians of opera. Peace.

That flag is to control how dark mode behaves when you force dark mode for a page. It's not for enabling dark mode on its own.

This is not the behaviour observed in Chrome (also Edge, I just tested it) for the #enable-force-dark option.

Basically in Chrome or Edge, #enable-force-dark turns ALL WEBPAGES into dark mode regardless of whether they have a dark theme or not.

Have you ever refreshed a website using dark mode and seen short-lived white flashes during when it loads up? That's the browser's default white background showing up while the rest of the web content is loading. And when I use the #enable-force-dark option in Chrome/Edge, that background is flipped to black and I don't get epilepsy when I visit webpages.

For example, goto https://example.com/, click the Easy Setup button and choose "Force dark mode on pages". Then, change the flag to "Enabled simple HSL" for example and restart Opera and see how the example.com page looks.

If a page is in dark mode, you can right-click and disable dark mode for the page, which will add the site to the disabled list at the URL opera://settings/content/forceDarkMode.

For the flag, the most common option (if someone decides they need it) is the selective inversion of non-image elements so dark mode doesn't turn images white for example.