You can do this, but you will have to go through the CSS/SCSS files. What are you trying to achieve with 'banner'? Are you looking to replace the header with a background image? (You can get the source files here: Learn how to download and use new screen profiles and layouts in RoboHelp
In the SCSS, you change the banner on lines:
44: all devices - This is where the background color is set, so the optimal location to set your banner.
577: mobile only
1099: tablet only
1425: desktop only
Thanks Willam. Yes - we have a customised banner that is part of our organisation's standards. I'll get one of the developers to help me.