We focus so much on making things functional, reliable, scalable and whatnot that the aesthetics are often left for better times. Why should they be? After all, user experience matters the most.
Did you know you can change the background image of your Frame instances for both App mode and Desktop mode?
For App mode instances log in to your Sandbox, open up File Explorer and navigate to C:\ProgramData\Frame\.
Replace the "app-business.png" file with the image you wish to use as your background on your instances. The image file must be renamed into "app-business.png" and placed in the same C:\ProgramData\Frame\ directory which will overwrite the existing file.
Publish the Sandbox image, and the next production App mode instance will display the new background.
For the instructions on the Desktop mode, follow through the KB-8847 How to Change Background of Production Instances.
More on Frame is in Frame Documentation.