Where did you add it though? Needs to be inside the site block (e.g. just below root)
file_server is required to serve files in Caddy v2, it was implicit in Caddy v1.
You should right-click on the rendered page and click View Page Source, and compare the two.
Caddy’s file_server doesn’t modify HTML content by default, so you must have something else going on. Probably a CSS file not being loaded or something. Check your browser debugger’s Network tab to see if everything loaded properly.