In this case there are pngs used to display the items.
I dunno exactly, because I found all the default themes extremely ugly.
You could have a look at
https://stash.reaper.fm/ to find the old theme.
You could also have a look at "...\Users\<your username>\AppData\Roaming\REAPER\ColorThemes" (Win7 path),
rename e.g. "Default_3.0.ReaperThemeZip" to "Default_3.0.Zip",
unpack it,
search for the graphics "item_bg.png" and "item_bg_sel.png",
change them to your likings,
put all you unpacked back into a "Default_3.0a.zip",
rename it "Default_3.0a.ReaperThemeZip" and
put it back into "...\Users\<your username>\AppData\Roaming\REAPER\ColorThemes".
-Data