Hi! You do not have CLI access in shared hosting, if you choose dedicated, yes.
What I can do to allow edit to css or any other code is you indicating your fork on GitHub and I will update from there, you handle your custom code and I push the updates.
Emails are sent via MailGun, I set up 1 account @masto.host for each instance and the cost of notification emails is included. I can also use any other SMTP that someone provides or authenticate with MailGun dif domain
@Laraby :) to make a user admin you should:
RAILS_ENV=production bundle exec rails mastodon:make_admin USERNAME=username_of_admin_user