Shared Settings, accessed from the Settings menu, allow you to set up default colors and images for use across your forms. You can always customize each form on it's own, but Shared Settings gives you that initial standardization for your brand. You can also enable the option for donors to be charged the processing fee for their donations.
Go to Settings (the Gear icon) on the right top side of the the navigation menu.
Click on Shared Settings on the left side.
Add your organization's brand color, your logo, and a standard background image to be seen on the desktop version of all your forms.
Enable the option to Charge Processing Fees, and choose the appropriate percentage.