You can add custom CSS inside the theme options panel at Advanced → Custom CSS. This is the preferred method of adding CSS, since it will be saved if you update the theme.
Storm has one main stylesheet style.css in the theme root folder, you can make any adjustments in this file. The file is split into sections by numbered comments and the file has a table of contents at the top to help you find the right section. We recommend adding custom styles in section Custom at the very bottom of the CSS file. Note: when updating the theme you should make a backup of your changes, this file will be overwritten.