Quicktime buttons disappeared, but they still function as 'normal', you just can't see them

I recently downloaded a video to use for teaching kids, from a reputable website which I've used many times & never had issues with (as others have), but when i went to play the video all of the quicktime control buttons had disappeared. All that is visible on the Quicktime control bar is the volume slide level control & the video progress 'bar' & times (I've added a screen shot). The same happens now with all my videos.

All the buttons still work though. So for example if i was to click where the play button would be, etc the video will play & function as 'normal' except you just can't see any buttons.

I have no idea whats happened, as far as i am aware i have not adjusted or tampered with any settings or preferences, and all was fine until i downloaded this video (though that fact would just be coincidental)

I have a MacBook Pro (13-inch, Mid 2012), running OS X El Capitan 10.11.5

help would be appreciated as it annoying & i can't just uninstall & reinstall as its now apart of the OS X