CSS can be used to change how elements on a website are displayed, overriding their default appearance. If you put your own CSS here, after VSys Web is synchronized, all web pages generated for your site will put your CSS after the default CSS, letting you modify how they look and are laid out.
We can't help you design or edit your own CSS here. If you have a volunteer or staff member who's a good web designer you're welcome to be creative, but custom CSS for VSys Web isn't part of your support agreement.