Shortcuts for zooming

Hi, I have a tablet PC for drawing and the program works fine, except for one small, but super annoying thing; Zooming.

When I draw on my computer (Surface pro4), I disconnect the keyboard and don't need a mouse, but then there is only the option: to zoom in, basically. I can't zoom out with the tool- well, at least as far as I could gather so far :P I might be wong here, since I'm new at the program.

Anyway, is there any way to get key shortcuts for the zooming? 'Cause I can't seem to map it like you can in PS (and I wouldn't expect it to be necessary for anything other than this, pretty much). I have a plug-in this would work with, and even with my keyboard plugged in, a touchpad is absolutely TERRIBLE to zoom with. I tried the ctrl ++ like in PS, but that works with the brush size in stead (which is fine). 

I hope someone can help XD