[Chore] Update Hover/Select Background Colors in Select Dropdown

[kpmurphh]

When working inside of a dropdown derived from a Select Form Field, the background of each component should be as follows:

• Default background (not being hovered or selected) - #FFFFFF
• When hovering over an item - #F6F6F6
• When an item is selected to be added to the main form field - #EBEBEB

See picture for example:

[Xenopotamus]

@kpmurphh @stolve suggest we update this ticket to make the selected item more visually prominent and distinct from the hover color of #F6F6F6. In Mobile Savings Center, we had proposed (for dark theme) using the primary teal color (#00838F) with white text to show the selected item.

Example:

@kpmurphh As part of this ticket, we should also be specifying what the Dark Theme colors should be for default, hover, selected states.