Many of the colors used on your site can be customized in the Colors section of the Basics tab on your dashboard (Design > Basics).
To set a color, click on the Color text field. Either select the color from the color selector, or type in a new value (in hex form) for the desired color.
Click Save configuration. The color will be applied to the site and saved to the theme selected.
If you want to reset the colors back to the theme default, simply click on Revert to defaults and then click Save configuration.