GujjuGang7 on Reddit found this, saying:

Found this link while looking through the upcoming theming engine (Union) repository. It has mockups for several core apps (dolphin, Kate, konsole and more) and general components such as modals and titlebars.

KDE contributor Manueljlin would like to remind you:

hey folks, it's really early still. we didn't even properly show it at Akademy. there's no design system to properly back it up yet - only some tokens and components that are definitely subject to change. please keep that in mind