Black background on any app window Suddenly my computer (iMac 27" late 2012) screen appears like the screenshot I am attaching (happens with all apps)
Could it be just software issue that can be fixed or looks like a hw problem?
Looks like graphic distortion.  If resetting NVRAM and resetting SMC don't resolve the issue (and you don't have any other symptoms, e.g. crashing applications), visit the Genius Bar.

P.S.  If you feel up to the challenge, run Apple Hardware Test and see what you find.  Then bring your results to your Genius Bar appointment.