manual/06-themes.md captions this entry “Matte Black”. It lives at themes/matte-black on the quattro branch.
That chapter says each theme styles the desktop, terminal, neovim, the btop activity screen, Chromium, and the Omarchy shell: top bar, menu, notifications, OSD, and the lock screen. It also says Obsidian needs the Omarchy theme selected in the app under Appearance > Themes.
The background, accent, and foreground values on this page are copied from themes/matte-black/colors.toml. The preview is themes/matte-black/preview.png from the same directory.