There has been an issue till 2 -3 weeks before. But I don't expect it to happen now.
I tried this link
https://www.zettlor.com/employer-demo and switching to dark and light mode. It stays in light mode without any flicker. Everything seems to be fine. I even tried and tested by opening the app side by side(the scenario in which the bug was there earlier).
Which browser are you using(just in case it's a browser specific thing).