This is a very simple emacs mode for Isabelle (.thy) files. It provides only some syntax highlighting.
The intended use is in combination with (LSP-isar).
If you use quelpa
elisp :location (recipe :fetcher github :repo "m-fleury/isar-mode")
Otherwise clone the repo and add it to the load path.
The mode started as a fork of (simp-isar-mode), but later changed as most of the syntax highlighting is provided by Isabelle's LSP server.