Most of your site's colors and styles will be controlled by the Theme's respective Cascading Style Sheet (CSS).
To modify your site's current style sheet, use the following steps:
From your online Store Manager,
Go to Settings > Design > Themes & Styles.
Click on the Edit look/Color (CSS) button located along the top right corner of the page.
You will then see a listing of each style controlled by the style sheet. Clicking on the Action > Edit button on any of these will allow you to make changes to the selected style itself.
After making your changes, click Save and review the changes on your site.