How to create and send an HTML formatted eMail using CSS?
With some experience in that field I must say designing 'nice' html mail templates can be difficult – no matter what system you use to actually send your mail. The reason for this is that there are quite many different email clients – from full blown local desktop computer programms to mobile apps on smart phones and feature phones – and a virtally countless number of online services –– all behaving slightly different. Some email clients do accept css3, some don't interprete css at all, some only accept inline css.

So I suppose you probably either have to start digging into that – or use the methods that are rather 'safe' to use as offered by various services and/or modules.