Don't think these styling changes are significant enough for you to want to include additional stylesheets.
If you're keen to avoid "soup," you might want to look at combining classes. So you might have
on one page, and you might add to it on another:
<div class="maincontent textcontent">
In the simplest case, maincontent would do the bulk of the styling, and textcontent would left align the text.
Hope I'm making some sense here... !!!