Frappe Books is a desktop accounting app, one that has a modern UI. It's simple, well-designed, and free. Usually you have to give up one of them, and if not there's often a catch. Enter your credit card details. With Frappe Books there is no catch.

Invoice Neko is a self-hosted open sourced invoicing system built on a modern backend with a focus on delivering a good user experience throughout the application

Fluid responsive design. Elegantly scale type and space without breakpoints.