The background colors in custom.xml are used by the Full Browse layout, as that is a solid color, but not by the default layout for Fire App Builder.
For the default browse layout, an image is currently used for the background to accomplish the fade around the image in the top right. An immediate fix for this would be to create a PNG and name it bg_generic.png. Place this in the custom.xml file of the TVUIComponent to override the existing one. Unfortunately, the PNG will need to be created similarly to the one we currently have, i.e. with the top right corner missing, otherwise it will cover the image of the selected content.
In a future version (post-2016), we have plans to make it easier to customize the colors and not have to work with a background image.
Dec 13 Update: The Fire App Builder documentation has been updated: