TeXstudio: content-assistant popup window is unreadable
Solution 1:
I was able to resolve this by changing the style.
Go to Options > Configure TexStudio > General.
Under Style try something besides GTK+ or default. I had success with everything but these two.
Solution 2:
You can also solve this by unchecking "Ignore most system colors" on this option tab (advanced options have to be enabled to see it).