The image doesen't supports Transparency and i have come to this conclusion after this test:-
These is my Theme devlopment/tweaker window with a search of "media background"
This the color for all the 4 Lists (Doesn't matter as i have fill mode to normal and zero)
So, i tried a completely transparent image:-
Image:-
Look:-
and a color with no transparency for item_bg_sel:-
Images:-
Look:-
and the media item background lookes transparent and colored as aspected.
but when i tried any color with decreased opacity, the selected media item background doesn't changed.
Image :-
Look:-
it's like reaper ignores the image with transparency in it and replaces the image with the item_bg