If you look into Perl Monks Approved HTML tags (it's automatically linked when you comment) you'll see a list of allowed attributes per tag.

tbody   width, align, valign, colspan, rowspan, bgcolor, height, class
td      width, align, valign, colspan, rowspan, bgcolor, height, class
tfoot   width, align, valign, colspan, rowspan, bgcolor, height, class
th      width, align, valign, colspan, rowspan, bgcolor, height, class
thead   width, align, valign, colspan, rowspan, bgcolor, height, class
tr      width, align, valign, colspan,          bgcolor, height, class
[download]

you can go by class or bgcolor,

> using the user-selected stylesheets?

OK that's a bit trickier ... are you talking about themes?

The monastery has a default set of CSS which is altered by theme. Users can overwrite whatever they want in their settings.

I'm not aware of classes for tables, but you can look into the HTML of existing tables° or the CSS in the settings section.

And if they don't exists, I'm sure the gods will grant you a wish to create them ... :)

edit

°) Saints in our Book is using <tr class="highlight"> for that.