I'd normally recommend adding additional CSS and JS at the web property level (your website or whatever property the form is actually displayed on) whenever that's possible rather than adding the CSS and JS to the form itself.
It is possible to add CSS into the Custom CSS piece of the Marketo Forms and to add JS into a Rich Text element on the form itself (although this is kind of frowned upon from a dev perspective - maybe more of a "last resort" kind of tactic).
The advantage of adding the CSS/JS at the web property level is that you are able to use the form in a more global context should you want/need to push that same form to a different destination in the future without needing to clone the form and make changes and end up maintaining multiple copies of the same form and a branching network of rules and lists to handle the differences between the iterative forms.
The reasons that I've normally heard for doing the CSS/JS on the form itself is that you don't have access to the web property (maybe your website) and/or that it take forever to get anything done on your website and you'd like to be more in control of the pace and ability to change things as needed. If that's the case, adding the code to the form can be a good short-term solution to work around a real business problem, but I'd almost always recommend finding a better solution to the underlying business issue than continuing down this path b/c over time it can lead to a big mess of redundant forms that are difficult to organize and manage and present a real obstacle when something changes with your branding or something like GDPR comes along where you've got a update all the forms with something new. Global forms is a clear winner for future-proofing your stuff and make situations like this much easier to manage as well as being a more elegant solution within your campaign architecture in Marketo.
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