I ran into a similar problem.
Easy to replicate clean install, install your terminal theme, then attempt to in-page edit, and you will receive an un-styled version of the theme.
My console error log is:
GET https://my.domain/index.php/packages/dahmen/automad-terminal/dist/terminal/terminal.grid.css net::ERR_ABORTED 404
(index):20 GET https://my.domain/index.php/packages/dahmen/automad-terminal/dist/terminal/terminal.min.css net::ERR_ABORTED 404
(index):19 GET https://my.domain/index.php/packages/dahmen/automad-terminal/dist/normalize/normalize.css net::ERR_ABORTED 404
Mine has to do with /index.php/ being appended to terminal css locations, if I remove index.php it loads css. I was playing with the config and found pretty url options in const.php i removed the append of index.php if not using pretty url's and now the CSS loads but the in-page editor bottom menu buttons now do not work.
When doing some testing, I assumed it was my usage of Caddy as a web server (thought my URL rewrite config was incorrect) but now this is unclear...
It would be very interesting to hear what you have to say in regards to the terminal theme 404's and also what your thoughts on Caddy as a web server for Automad. (I've written a guide but until it is working 100% I don't want to publish.)
(Automad or Die)