For styling options beyond the built-in settings of the platform, you can now add custom css to your configurations.
1.1. In the platform, from the top menu, select Engagement, from the white bar underneath the top section, select Personalization, and then Site Optimization.
1.2 From Site Optimization, select the Preview option from the left menu. Then select the button Customize colors and fonts.
Once you select Customize colors and fonts, you will see the Custom CSS window:
With background color selected, but before applying CSS customization:
2.1 In the tool add CSS to the element you'd like to edit, then select Apply (at this time, custom styling will not render in preview, but should display on deploy):