I had the same issue! The "Force Dark Mode" option used to be under Settings > Appearance, but it seems to have been removed in recent updates.

Since you've already enabled opera://flags/#enable-force-dark-from-settings, try this workaround:

Go to opera://flags and search for "Force Dark Mode for Web Contents".

Set it to "Enabled" instead of Default.

Restart Opera.

If that doesn’t work, you might need to use a dark mode extension from the Opera Add-ons store until they bring the feature back. Hope this helps!