I have Juicebox Pro, so I'll try the web fonts customisation with that first (I'm assuming it works in the same way as Showkase, of course).
Please see Juicebox's Using Custom Fonts support section.
One more question: is it possible to have multiple galleries on one page, as with the Juicebox demo showing this implemented with text links to each gallery at the top of the page?
If you really want to have multiple galleries embedded in the same page (like in this example), then you would need to create your galleries on your computer using JuiceboxBuilder-Pro and then embed the galleries into a Basic page manually using the baseUrl method of embedding documented here. (Showkase displays each Juicebox-Pro gallery on its own separate page.)
If you want to group together several galleries, then you can create a Gallery Index Page and then list whichever galleries you like on the page. After creating a Gallery Index Page, just edit a gallery page and in 'Site Navigation -> Group Under', select the name of the Gallery Index Page. Each gallery listed under the index page will be represented by a thumbnail image which, when clicked, will open the selected gallery on its own page. You can have as many Gallery Index Pages (and galleries) as you like.