Hello,
If I'm not mistaken, what you tried to do was to obtain something similar:
I have also tried to replace the textures of the DropDownList, and I achieved at this result. In fact, there is no code interfering with the texture. Your issue with the blue background is due to this texture: Interface/Tooltips/UIFrameTooltipBackground
I can provide you the files with all the textures required for the replacement of the DropDownList,
here.
A little ago, your posts helped me to know which elements needed a change for accomplish this. I hope this could help you.