Thank you for the attached picture.
I went through the RadSplitButton style, however it does not have any styles for the content placed in its dropdown. With this in mind, the described background most likely comes from whatever element is placed in the DropDownContent of the RadSplitButton.
For example, if the DropDownContent is a RadListBox, the highlighting will come from the RadListBoxItem's Style. In order to remove it, you can extract and edit
the Style of the corresponding element that is placed within the dropdown of the RadSplitButton. If you need any further assistance with that, please share how the RadSplitButton is defined in xaml, which theme you are using and I will prepare a sample project for your reference.
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