First some info:
I use Manjaro KDE 16.10.3 with KDE 5.8.4
Themes I use are:
Look and Feel: Breeze
Desktop theme: Breeze light
Cursor theme: Breeze snow
Why? Because I like it. Plus I feel these themes are the true KDE themes, but I can be wrong of course.
In the attached picture you see what I mean:
Here you see my wallpaper (with conky), Google-Chrome and Virtualbox-manager all at once.
Especially Virtualbox is difficult to use. When I click the blue icon in the panel a small window appears with 2 lines of text:
Virtualbox-manager, and the virtual machine which is running at the moment. These two lines of text are barely visible, so choosing the right one is difficult.
Now I wrote which themes I use cause I think this might be the problem, but on the other hand, aren't they also made to be used? I can change them but I don't really want to. I like the bright windows I have now.
Cold there be some setting somewhere which makes the windows almost completely transparent? I can't find it.

[Edit] One more thing: I use Nvidia driver 375.26 at the moment on my GTX-760 card, but also with previous drivers I had this problem.
[Edit] One more thing: I use Nvidia driver 375.26 at the moment on my GTX-760 card, but also with previous drivers I had this problem.

You can change the transparency of a window by using the scroll wheel on its title bar (at least with the default settings).

The general window transparency is configurable in Workspace Behavior->Desktop Effects, click on the settings button for the "Transparency" effect (or disable it completely if you prefer).
Or, you can override it for specific applications/windows via Window Rules.

Btw, I don't think this is related to the configured themes at all.

I just tried that and it doesn't work at all. I see no differences between fully transparant and zero transparant.
What do you mean with the scroll wheel on its title bar? Is that the mouse-wheel cause I don't see any scroll wheel on a title bar.
Also, in my KDE 5.8.4 the desktop effects are in Desktop behavior. Can it be you are referring to KDE4.x?

DeMus wrote:I just tried that and it doesn't work at all. I see no differences between fully transparant and zero transparant.

Did you try to disable the Transparency effect?
What happens if you disable desktop effects completely? (press Shift+Alt+F12 to do that temporarily)

What do you mean with the scroll wheel on its title bar? Is that the mouse-wheel cause I don't see any scroll wheel on a title bar.

Yes.
Moving the mouse-wheel while the mouse is over a window's title should change the window's transparency.

Happened to me once that I did that by mistake (looked similar to your picture) leading me to think I have a graphics driver problem at first...

Also, in my KDE 5.8.4 the desktop effects are in Desktop behavior. Can it be you are referring to KDE4.x?

No. In KDE4 it is a top-level entry in systemsettings.
But I'm running it in german here, I thought it would be called "Workspace Behavior" in english, not "Desktop Behavior".
Sorry, I remembered it wrongly.

wolfi323 wrote:Did you try to disable the Transparency effect?

I did not have it enabled in the first place.

What happens if you disable desktop effects completely? (press Shift+Alt+F12 to do that temporarily)

That makes a huge difference. Now the picture is okay again. Can it be caused by one effect, or is it the whole effect structure which causes this?
The effects I have switched on are:
Background contrast, Glide, Maximize, Scale in, Sliding popups, Dim inactive, Slide back, Slide.
I will experiment with those to see if I can find the one.

What do you mean with the scroll wheel on its title bar? Is that the mouse-wheel cause I don't see any scroll wheel on a title bar.
Yes.
Moving the mouse-wheel while the mouse is over a window's title should change the window's transparency.

I can scroll as much as I want on the titlebar but nothing changes.

Happened to me once that I did that by mistake (looked similar to your picture) leading me to think I have a graphics driver problem at first...
Also, in my KDE 5.8.4 the desktop effects are in Desktop behavior. Can it be you are referring to KDE4.x?
No. In KDE4 it is a top-level entry in systemsettings.
But I'm running it in german here, I thought it would be called "Workspace Behavior" in english, not "Desktop Behavior".
Sorry, I remembered it wrongly.

Okay, sorry for my remark. Didn't know you had to translate it. My apologies.

Had to switch off effects completely (shift-alt-F12) to get a good picture again. Just de-selecting all effects wouldn't do it.
But this does mean something is wrong, right? I should be able to use effects, shouldn't i?
Who else is facing the same things I see when using effects? Did you manage to stop it? If so, how?

I stopped using shifr-alt-F12 cause it caused an other problem, see thread: https://forum.kde.org/viewtopic.php?f=289&t=138197.
I did a re-install of the OS, unticked all but one desktop effect and I can see everything well now, no more picture in picture in picture, I still have the one effect I like and I don't need to use shift-alt-F12 anymore.
Thank you wolfi323 for your help.