You don't have the same control over the modal forms themselves. The portals basic form definition does have a place for custom JavaScript (and via JavaScript you can adjust the CSS), and on fields you can also do some additional stuff by adding form metadata records, and you can also embed HTML Web Resources on the Dataverse Form that is being used and have it render (tickbox on the portal form definition).
Also, if you really wanted to you could build your own modal popup using HTML, JavaScript and the Portals Web API.
