Backstage is the design system for the company Backstage.io, they call themselves an open platform for developing portals...and I have no idea what that means. What I do know is that they've open-sourced their design system for anyone to adjust and use. So if you're looking for a solid, heavy, dark themed design system then this is a fantastic starting point.
This free Figma design system includes everything from the usual fonts and buttons to cards, chips, states, filters, error codes and more, it really is that robust.
To access the Backstage design system; simply click the big red button below, then click the duplicate button in the top right corner.