There are two color scheme options in Bonusly: Custom and Default. You can choose which color scheme you prefer from the basic settings page:
You can own custom color scheme by clicking the 'Custom color scheme' radio button below the Logo option. Creating a custom theme gives you full control over the header, the header text, the main brand color, the body, and the sidebar text color.
(Once you're done, remember to scroll down and click the 'Save Settings' button.)