You have the ability to change the Client Portal Background Colors in the system.
You will go to Manage > Company Settings.

Once here, scroll to the bottom of the page to the Company Branches section.
To edit you will select the Pencil Icon for the branch you want to edit.

​Once selected you will see, highlighted in red, the area to change the color options for the Customer Portal.

Once the colors you would like are chosen you will need to click the Blue "Update Branch" Button at the bottom of your screen.