Make a public API for application window color "themes"
Haven't you heard enough complaints about the dumb design choices you made for Office 2013 application window color "themes".
The additional themes in 2016 help. But why not open up an API for third party developers to make design decisions for color themes.
Then you could also create ports of the 2003, 2007 and 2010 color themes for people to pick from for use in the new application.