Adding my voice to the request for this feature! Having the ability to import like you can when building a Hub/Site would be great. I'd also just be happy with what I'm assuming would be the easier fix, just add maybe a dozen or so choices to the list of already available fonts (e.g. open sans, merriweather, merriweather sans etc). What's currently available is pretty limited. I'm currently building a variety of demo apps for my org to showcase ExB capabilities; the impact would be even better if I had a few more font choices.
I'm currently limited to AGOL in my org at the moment so Developer version isn't an option...actually, looking at documentation right now and maybe it is; will continue investigating that option.
