Drop down option hover css

I’m putting the finishing touches on my first custom theme, and I can’t seem to figure out the css for hovering over the drop down options in the settings menus (blue select in the attached image).

I figured this would be something like:

.dropdown option:hover {
background-color: #;

Can anyone shed light on this as the list closes when selecting the inspect button closes the drop down, making it very difficult to debug.
dropdown option highlight