I don't believe it's possible to customize the styles used for producing Emails.
You can access/customize the CSS stylesheet that's used to produce the application UI -- for example, you could customize the CSS file to change the appearance of buttons in the DocuSign application for your account. You can access the application UI's CSS file in the same place that you can access Resource files -- i.e., Preferences >> Branding >> (Edit Brand) >> Resources (tab). (If you don't see it there, DocuSign will have to enable your access to CSS for your account.)
FWIW -- even though it's possible to customize CSS styles used by the application UI, it's an error prone process that's difficult to maintain as DocuSign releases new product versions every few months. IMO, unless it's an absolute requirement, modifying the CSS file isn't a great idea.