The most important CSS selector is .tablepress as it applies to all TablePress tables on your site. If you don’t want to apply a styling setting to all tables, but just to certain ones, you can use .tablepress-id-N, where N is the ID of the table.

In most cases you’ll want to style a cell, so the CSS command will look like this:

.tablepress-id-N tbody td {
 property1: value1;
 property2: value2;
}

(Here, N has to be changed to the table’s ID, whereas property1 and property2 are just for demonstration and need to be changed to the CSS properties that you want to change.)

You can find a more thorough list of available CSS selectors in the Documentation.