Enforce logic on the Applicant page

Content

Ultimately, what I want to do is enforce a specific piece of logic on the Applicant page that would require a Resume to be uploaded

if a specific dropdown value is selected for our "Referred by"  field and "Referrer's email"  is filled.

I dont see how this can be done in TBE Recruit configuration directly, but wondering if they following can be done:

a.) in customizing the Careers website header and footer values, include javascript that would be invoked on submit

b.) similarly, wondering if we can apply CSS styles this way?

is there a set of best practices for doing this?