How can I hide soft buttons? I have a tablet with 3 soft buttons (Back, Home, Recent) with Android 7.1.  

Is there some way to hide them (and consequently use the full available screen), or at least dim them?

(They are disturbing when watching a video with my favorite old video player, which is not able to hide them.)
Navigation Gestures for Android, featured app on XDA Developers will work for you. Its free for most of the features. It hides the navigation bar completely and enables gestures to operate the phone. 

Refer to this page for details: Navigation Gestures on XDA