Is there a plasma desktop theme that has also the possibility to change its overall color and opacity in an easy way? About the same way you can in Windows 10 and Gnome shell? My feeling is that I really don't need all the different plasma themes. But one simple one with these options would be great.

There are a few plasma themes you can get that derive their colors from the application color scheme. Off-hand I know the Freeze and Colorbit themes do this, and I think one of the Breeze themes does as well.

As for opacity, unfortunately that is tied to the theme and I think to the elements used in the theme, and so may not be so easy to adjust. It would be possible to do with a little bit of scripting (possibly using a tool like imagemagick, or just by reading and writing the raw svg files) if you were up to that.

thank you for your reaction.
I think it would be a good thing if kde had a desktop theme that you could easily change to your taste. I think I will drop it in a wish list at the VDG. I will look in to it.