I believe the Inbox Items Field & List Details Field Background Colors are not behaving as intended.
Changing the background color for either section affects the background color for both, or that is what appears to be happening at the moment. The List Detail background color does not seem to affect the background color of the select list.
I’m having the same issue for the Inbox Block. The ITEM FIELDS customization only allows me to change background color, and whichever color I choose, the background of both the List Item and List Item Detail fields are changed to that color. The selected item in the List Item (left) column is a lighter shade of the background color. There is no way I can see of easily setting the text color either - both states use the same selected item color which can be an issue with contrast, especially on the selected field.