The Theme Selector features a gallery of pre-designed color schemes that you can apply to a campaign to change its appearance.
Open the Style Panel, then click the Change link at the top of the Style Panel.
Click a theme’s thumbnail and your campaign will refresh to give you a preview of that theme. You can preview as many themes as you'd like, but the theme isn't actually applied to the campaign until you click the Save button at the bottom.
Once you choose a theme, you can update the campaign's appearance further using the Style Panel and/or CSS.