To add CSS commands (Cascading Style Sheets), just go to the “Custom CSS” text area on the “Plugin Options” screen of TablePress and enter them there. They will override the default styling, so that you do not have to change any files directly (such modifications would be lost after each TablePress update).

Please note that you must not add the CSS code to the “Custom Commands” text field on the table’s “Edit” screen. That field is used for a different type of code.