Hi Danilo,
Thanks for using Syncfusion products.
We have Edited the style of the TabItem for changing the format of selected item polygon(the triangle at the right corner) which you can download under the following location.
Sample Link :
http://www.syncfusion.com/downloads/Support/DirectTrac/86619/86619_Ribbon_BackstageTabitem2114695762.zipAlso to set the Icon in BackstageTabItem we found a issue in ItemTemplate and have logged Issue report internally.we would updating the fix in the reported Incident #86619 through your Direct-Trac account.
Please let me know in case of any queries.
Regards
Surya Narayana