The most important selector should be .tablepress which applies to all tables generated by the plugin. 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;
}

(property1 and property2 are just for demonstration and need to be changed to the CSS property that you want to change.)

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