As to where to put it in Serendipity, there's usually three sources of CSS in a Serendipity blog, all of whom are being combined by Serendipity dynamically into one stylesheet.
- your theme's stylesheet, usually to be found at /templates/<YOUR_THEME>/style.css (but it can have a different filename, and there can be multiple .css files there)
- styles generated by the plugins you use (those are usually injected dynamically be the plugins, meaning there is no file you can edit
- in addition, you can (but don't have to) add your own /templates/<YOUR_THEME>/user.css files which Serendipity will automagically use
Most s9y themes will not style tables. That's because tables have a bad rep (which stems from them being misused for layout purposes), so most people are unlikely to use them because they have heard that "tables are bad" (which is not true for displaying tabular data at all). Also, it can be a hassle to generate and edit tables using most WYSIWYG editors, so it's not very common to even see tables in blogs.