This may just run so counter to Adobe's philosophy that it's immediately a non-starter, but, one of the appeals of Report Builder (as well as purely third-party platforms like Tableau and PowerPoint) is that, in Excel, I can set up a custom theme to control the overall look and feel of deliverables.
Even if this were clunky/complicated, it would be very nice to be able to get out of the Adobe-chosen green-based palette and font selection. Two ways this could work:
Customize the CSS
This would be the "Prezi model" or the "Google Sheets model" (or, heck, the "Wordpress model," for that matter) -- the ability to access the CSS (or visualization configuration file) that is being used to render AW. With proper documentation and some work, presumably, organizations could get deliverables tailored to their style guidelines, rather than being forced to live with Adobe's "style" guidelines.
Viz Configuration Interface
Call this more the "Microsoft model." But, providing an interface that lists out all of the visualization components/values and allows them to be tweaked:
Palette (there is clearly a palette of a sequence of colors used across different data viz options):
Panel heading font color
Panel description color
Series #1 fill color
Series #2 fill color
Series #X fill color
Series #1 outline color
Series #2 outline color
Series #X outline color
Page background color
Panel background color
Gridline color (including "None")
I realize this may be completely anathema to the Adobe philosophy, but it's an idea that I think would be embraced!