Please excuse me for omitting this question.
You can either hide the FillPrimitive of the button:
_btnHelp.ButtonFillElement.Visibility = ElementVisibility.Collapsed;
or you can set its colors to Transparent:
_btnHelp.ButtonFillElement.BackColor = Color.Transparent;
_btnHelp.ButtonFillElement.BackColor2 = Color.Transparent;
_btnHelp.ButtonFillElement.BackColor3 = Color.Transparent;
_btnHelp.ButtonFillElement.BackColor4 = Color.Transparent;
However, as expected you will loose the hover effects. One way to handle the hover is to have another image which will be shown when the button is hovered. You can use the MouseEnter and MouseLeave events to switch the images.
I hope that the provided information addresses your question.
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