I can definitely understand wanting to customize form select fields. At this time, it’s a known issue among browsers – they each have their own take on form styles. You can read more on this here: Dropdown Default Styling | CSS-Tricks - CSS-Tricks
For now, the best way to edit this is with custom code. Here’s a Codepen example of something you can build off of: https://codepen.io/chriscoyier/pen/pvfyt