I just installed KDE on top of Ubuntu, but when I installed "sweet theme (dark)", I noticed it was applied but window canvas remained the same. All the buttons, icons etc. were changed. Check this screenshot to get a better idea of what I am describing https://imgur.com/a/gjAA2D1
Things I tried:
- Changing GTK2 and GTK3 themes from "Application Style" but changes were not applied.
- Running System Settings as root and tried to change GTK2 and GTK3 themes, but the modules were not there.

Sweet being a third-party theme you should report this to its author.
Please make sure you have a correctly installed Plasma in the first place e.g. making sure you use the Kubuntu packages and have everything isntalled properly
How did you install the Sweet theme?
FWIW: I just tried the Sweet Theme (no mention of dark in the name, as it is a dark theme by default) and can't reproduce what you experience.
FWIW2: do not run GUI applications as root, there is nothing to gain from other than breaking your system