You can add styling within div and span tags within posts such as:
Kind of a pain, but works. You can do the same with text widgets. In order to change the style of the existing theme/template, however, you'd need to do a CSS upgrade. That's why WordPress.com strips out [style][/style] containers - if they allowed it then everyone could work around the $15/year CSS upgrade by putting their custom CSS fr the template into a text widget. Well, everyone with a clue anyway...