Hi, a new theme appeared in my site, it’s called “Foundation”, I never installed it, do you know something about that ? Thanks !
AI-generated summary
Users are discussing the appearance of a new theme called “Foundation” on their Discourse sites, which some did not install. HAWK clarifies that Foundation is the new name for the Default theme and replaces it if unmodified. patrickemin thanks for the explanation. RGJ shares screenshots showing Foundation appearing in newly deployed sites, though only “Default” is visible in their theme list. sam explains that Foundation and Horizon are new “core themes” shipped with Discourse, always present in the UI, and cannot be edited beyond components. kris.kotlarek notes a fix to hide these system themes from the UI for now, linking to a GitHub pull request. agent_kith asks what happens if Default is renamed to Foundation and reports that Foundation remains selectable even after disabling Default, while running version 3.5.0.beta8-dev(fcb943fa70). The discussion reflects ongoing changes and user confusion around theme renaming and visibility.
I believe Foundation is the new name for the Default theme and will have replaced it if you hadn’t modified Default. @jordan.vidrine can correct me if I’m wrong.
Ah yes thanks for the explanation !
This change is in flux over the next few days new installs will see “Foundation” and “Horizon” in themes which are “core themes”.
Foundation is the same default theme you have today.
These are themes that will always be there in the UI, are shipped with Discourse and can not be edited outside of adding components etc.. (no html/css overrides)
I made a quick fix so that system themes like “Foundation” and “Horizon” and their associated color palettes are not presented in the UI.
At least at this stage, when they should still be hidden 
What happens if I rename the Default theme to Foundation?
I have disabled the Default theme as an experiment, and thus cannot pick Default from the theme switcher now.
However Foundation is still available as a choice. Currently on 3.5.0.beta8-dev(fcb943fa70).