The v4.0.0 release notes say “Add Solitude, Last Horizon, and Lupine as new themes.” and “Add Last Horizon and Solitude themes by @HANCORE-linux”. This directory is themes/solitude on the quattro branch.
manual/06-themes.md does not caption this directory in its gallery.
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/solitude/colors.toml. The preview is themes/solitude/preview.png from the same directory.