When you are designing a WordPress Theme or trying to work with  a pre-made theme there are basic WordPress Classes that should be formatted in your CSS File.

The following list is the basic minimum classes that WordPress Generates when it spits back your pages.

Theme files will provide for much more fine grain CSS formatting and any plugin or widget will need additional formatting that can only be found once you install and implement the plugin.

Basic WordPress Classes Include

General Content Formatting
.aligncenter,
div.aligncenter
.alignleft
.alignright
.wp-caption
.wp-caption img
.wp-caption p.wp-caption-text

Menu Formatting
.categories
.cat-item
.current-cat
.current-cat-parent
.pagenav
.page_item
.current_page_item
.current_page_parent

Widget Formatting
.widget
.widget_text
.blogroll
.linkcat

Again Remember that each Theme and Plugin or Widget will have its own classes or IDs that need to be addressed.  The easiest way to understand what CSS formatting is needed to support an individual plugin or widget is to enable it and then read the source code around the widget to understand what steps are necessary and which hierarchy you will have to use to define element styles.