There are four key sections of My Tickets that are templatable: receipts, tickets, the opt-out form for email notifications, and the ticket verification screen used by ticket-takers. They’re all templatable in the standard WordPress way: take a copy of the file from
/templates/, place it at the root of your theme directory, and edit it until it does what you want.
By default, the templates enqueue their own custom CSS. You can use that CSS and extend it, or create your own; there are no limitations – the templates include their own CSS, so you simply edit the template to include something different, if that’s what you want.