The dark theme is not working in Opera after upgrading to latest version v109 (specifically 109.0.5097.45). I downgraded (using brew) it back to version v108 (specifically 108.0.5067.40) and the dark theme worked fine.
I am using Mac M1 (v14.4.1) and my system is upgraded to latest. I tried disabling all extensions in v109 but that didn't work. Downgrading to v108 resolved the issue.