ShortPoint will support modern pages and sites soon
#1: ShortPoint Page Builder - Inserter
#1: ShortPoint Theme Builder - SharePoint Branding and Site Customization
Currently, the only supported way is with color palate and you can add a background image.
ShortPoint SPFx is now in beta. Please send us an email to firstname.lastname@example.org if you want to be part of early adaptors team, and help us during testing.