Thank you for writing.
I can confirm that the RadSplitButtonElements are rendered with the default color from the theme not with the ribbon color - I have logged this in our Public Issue Tracking System - PITS. You can track its progress on the following link - Pits Issue.
I would like to recommend to use a RadDropDownButtonElement instead of RadSplitButtonElements.
The RadSplitButtonElements renders correctly with Ribbon Color - please, refer to the attached screenshot.
I have also updated your Telerik points for bringing this to our attention.
I hope this helps.
All the best,
the Telerik team