Before directly overriding the CSS, check to see if the look and feel can be adjusted by Fine Tuning the Theme.
This document explains how to properly override the theme CSS. Following this technique will make sure that you changes are not overwritten during a theme upgrade. First navigate Appearance and then CSS as shown below.
This will open up the editor as shown below. Place your style overrides into this file and then save.
After making an edit to your CSS, make sure to use the reload icon on the admin bar of your StoreFront. This forces the browser to refresh all the assets on your page including the override CSS.
