Html emails use a lot of CSS. It relys on it to be honest when it comes to formatting fonts, colours and sizes. BUT... don't list .CSS at the top of theming or styling of templates.(like in html web pages in the head tags) Only do this for fonts etc.
For styling and shaping templates/designs. use inline css. As it's more cross email provider supported.
NEVER use DIV's or if you do only for like centering and things. Emails are not updated since the 90's so Div's aren't supported in most email clients, some will read it fine some will read it but not display it correctly to the eye.
Use tables in emails. it's like the opposite for web design tbh. To get an example of how to do one the best is to sign yourself up to a retail online website for a enwsletter await the first one and view the source code. I bet you it's done in tables as it's easier to control within email clients cross supported.
Sorry, this has going out of proportion to OP thread.
I remember a website like emailmonkey or something. They can supply you with free themes to work with.