To find the correct css properties go to the preview of your newsletter and use the browser's inspector to find the right properties.
You could also edit the source code (button in your editor) when editing your newsletter. There you can inline your own css (style="poperty:value;"). Please note that this will not work for content that is aded dynamicly.
Please, take also a look at Acymailing version 6. The drag & drop editor will make your life a whole lot easier.