Simply run alps with sourcehut theme:

% alps -theme sourcehut -debug example.com
2022-04-16T13:06:22+02:00 - Configured upstream IMAP server: imaps://imap.example.com:993
2022-04-16T13:06:22+02:00 - Configured upstream SMTP server: smtp://smtp.example.com:587
2022-04-16T13:06:22+02:00 - Loaded plugin "base"
2022-04-16T13:06:22+02:00 - caldav: failed to discover CalDAV server: discovery not yet implemented
2022-04-16T13:06:22+02:00 - carddav: failed to discover CardDAV server: carddav: domain doesn't have an SRV record
2022-04-16T13:06:22+02:00 - Loaded plugin "viewhtml"
2022-04-16T13:06:22+02:00 - Loaded plugin "viewtext"
2022-04-16T13:06:22+02:00 - Loading theme "alps"
2022-04-16T13:06:22+02:00 - Loading theme "sourcehut"
⇨ http server started on [::]:1323
2022-04-16T13:06:28+02:00 ERROR Error occured rendering error page: html/template:error.html:14:11: no such template "foot.html". How meta.
2022-04-16T13:06:28+02:00 ERROR html/template:login.html:43:11: no such template "foot.html"
2022-04-16T13:06:28+02:00 method=GET, uri=/login, status=200

Maybe it's because it doesn't take alps theme as a base?