I do web development in intellij, vs code and atom. Those editors have dark themes and I am going back and forth between them and the browser window. On my unity desktop I found it convenient to manually dim windows (like they are inactive) with alt + mouse wheel. I cannot find a way to do that with kde. Is it possible?

Such window actions are configurable in System Settings -> Window Behavior -> Window Behavior -> Window Actions.
(in kde 4 at least, though I'm pretty sure it hasn't changed in kde 5) checking the options for Mouse wheel, the closest option I see is Change Opacity.
Another option is the Invert effect (enable in System Settings -> Desktop Effects -> All Effects, to invert a window the default keybinding is Meta+Ctrl+U.

'd recommend proposing this in the Brainstorm forum