The element that holds (the sidebar that has "Folders" on it) is actually a DropDownButton which is shown when you minimize the OutlookBar. The easiest way to change that button default background is to use our NoXaml binaries and merge the xaml files of the controls you will refer in your project, you can find more information about this approach in this article. Doing so you will be able to edit any part of the style that you want, like in your case the Background of the "MinimizedContentElement" button.
In your case once you merge the necessary xaml files you will need to find an element with x:Name="MinimizedContentElement" and change its Background property.
I have prepared a sample project demonstrating this approach for your convenience.
I hope this information helps.
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