So, Decker doesn't use standard Windows format fonts, it uses its own font format. So if you're hoping to add like times new roman into Decker for example, you may be out of luck. But there are extra fonts you can add into your deck, or you can make your own if you want!
In the examples folder in Decker, there's a deck called fonts.deck that has a lot of explanation about how fonts work, as well as having a bunch of extra fonts you can add. In order to add the included fonts to your own deck, you can drag the fonts.deck onto Decker while you have your own deck open, and it'll pop open the Font/DA Mover that lets you copy fonts from the deck into your own. If you open fonts.deck itself, you'll also find a Font Editor button, which you can use to create your own custom fonts.
If you're looking for more premade fonts than are included with fonts.deck and aren't keen on making your own, then you might want to check out Coral Nulla's port of the Not Jam Font Pack into Decker.
There's also a few extra fonts in the decker-fonts github as well as a while back IJ ported a bunch of old Windows 95 fonts into Decker.
Hopefully this makes sense.