All the control buttons at the top of open windows are blank

On all my open windows (not only Safari but system prefreneces etc too) the little control buttons in boxes are just blank - back/forward arrows , reply/forward commands on email. It's all a guessing game! At one point I had very little memory so backed up and removed some stuff hoping they would come back as maybe the system got rid of them to save on memory but the problem persists.

Any help would be much appreciated! Also at the moment my safari won't minismise, Only have a red or green button and the middle (usually yellow) one is grey.Not the greatest problem I've got at the moment though!

Thank you 🙂