I'm using gmusicbrowser version, downloaded from github, on Arch Linux. I've been using earlier versions for many years.

In most ways, the gtk3 version looks and works fine; however, there are some icons that are missing. In particular, the main "Recently played songs", "Play", and "Next song" buttons don't have any icons. Hovering over the space where the icons should be, shows a tooltip with the above descriptions, and you can click on the space to trigger the action. So the functionality does work, it's just the icons that aren't there. They are missing in several different layouts, not just the custom one I normally use.

How am I supposed to get these? I think that I have normal themes installed on my system, and I couldn't see anything about this in the FAQ.

I had the same problem, after updating to LM22 (desktop) and kubuntu (laptop).
My solution to get the icons back is to change the symbol theme from "Standard" to the downloaded theme "Color", but other themes work fine too.
Just an other hint: With the layout AlexC base/compact the Album takes more than half of the screen.
My solution was to replace in AlexC.layout file "HBcenterup = _Cover" by "HBcenterup = Cover".