Emails are not websites, and while the design should be appealing, it should be kept somewhat simple. This is because emails are opened on a wide variety of browsers and email clients, and therefore it makes sense to keep the design basic so that there will be less chance of irregularities or errors occurring.
Firstly, can you use CSS? No no no. Email design needs to be trundled back to the good old days of tables and inline styles. This is due to the majority of email clients stripping out any CSS reference you make and therefore breaking your design. It may be ‘old school’ but it is the best way forward when it comes to creating emails that work.
Also, worth noting... try not to nest your tables, stacking tables instead creates an email that is easily editable and updateable, which going forward will save you a lot of time. And while we’re talking about tables, do not split or merge table cells. You may do this to align some elements in one section of your email, while unwittingly distorting other content somewhere else. It isn’t a reliable source of formatting so try to avoid it where possible.