You can see by yourself at https://triliumnotes.org/

It's FOSS, it's web so you can use it hosted, or local first as an electron app, or both and then they will sync together.

It is NOT a UI for a folder of markdown files, because that's silly when you expect from your system to hold relationships, metadata, rich note types, notes to coexist in multiple places, etc. Since it's FOSS, and since you can sync your notes real-time and distributed across machines, there's nothing wrong with this.

You can use the web version on Android as a PWA, but it won't sync offline. There are workarounds to run a local server on your device for that use cases (not ideal in terms of user-friendlyness, but gets the job done).

You don't need to pay anything to anyone if you host it yourself or if you keep it local. There is no official hosted plan, some people offer to do that for a tiny fee at https://www.pikapods.com/ (never used them, some people say they are decent).

Can trillium store all files in markdown/plaintext?

Content is stored in a SQLite db (with options to export to markdown & al.), Trilium is open source, so there's no lock-in and you get the best of both worlds.

How is the theming by trillium? I use a light tan interface because it is much easier on my eyes, personally than high contrast white or eye-straining dark themes.

You can totally reimplement the whole UI if that's your thing, everything (or close-enough) is a note in Trilium, including themes and other JS/CSS notes that will override or extend parts of the application, like add-ons would elsewhere.

So, I recently abandoned Trilium, because it's very half-assed.

It stores data in a database on your local file system, and you can export as markdown.