Since version 3.2 it is now possible to add custom fonts to your templates. Doing so is quite simple and can be done on any block within the uiBuilder.
Adding a font from google
Using hosted fonts is the easiest method of adding custom fonts with uiPress. Select the block you wish to change the font on and navigate to the block styles and then down to the text style stack.

Select custom in the font select and enter the URL to the font and the font family. The url comes from the script tag that google gives you as you can see in the image below.

Adding a local font
If you have a self hosted font that is already enqueued within the WordPress admin then all you need to do is set the custom font family.
If you need uiPress to load up the font then it needs to be a path to the css file containing the font face. It should look something like the below example.
@font-face {
font-family: 'YourFontName';
font-style: normal;
font-weight: 500;
font-display: swap;
src: url(pathToYourFontFile.woff2) format('woff2');
}