How to use Mouse Buttons 4 and 5 for forward and back?

[tunc]

I can't figure out how to use these buttons for forward and back. I found this thread (https://vivaldi.net/en-US/forum/vivaldi-browser/10215-disable-4-and-5-button-on-the-mouse-possible#55628) which implies strongly to me that I'm missing something. Has this feature been removed, or do I need to enable it somewhere? I'm using two Logitech mice on OS X, and these buttons work as configured in all my other browsers, as well as across the operating system (file exploring, etc.), but they don't seem to work for me in Vivaldi. It isn't a big deal, as I like the rocker gestures and mouse gestures, but I find that I really would like the same consistency for these buttons everywhere now that I've gotten used to using them that way. Thanks for any thoughts, DW