To use a custom font add a new font to addons/your-addon/fonts where your-addon is the name of your addon. It can then be used in your stylesheet using font-family: FontName. Note that you do not include the file extension. For clarification; in this example the font would be called FontName.ttf.

Note: If you load your stylesheet using StyleSheet.Load("ui/your-stylesheet.scss"); fonts do not hotload and therefore you will need to restart your game before you can use the font in your addon. Append a slash to the path to enable hotloading like this StyleSheet.Load("/ui/your-stylesheet.scss");