In this tutorial, we explore the key settings necessary to dramatically alter the appearance of the combo box. The combo box can be a challenge to customize as it comes with a myriad of settings. The goal of this tutorial is to make the customization process as simple as possible.
https://dev.epicgames.com/community/learning/tutorials/bO3J/manually-styling-a-combo-box-in-unreal-engine-5-4