Hey, you can change the entire theming of your desktop - in Raven (the side-panel, with notifications and applets, click on the bell or speaker symbol to open it) - at the top right hand corner, there is a gear icon, which will open the settings view. Here (under the heading "Appearance") you can choose a number of different themes.
I've just played around, and it seems that the only one that effects the active / background marking (without totally changing the theme) is IconTheme to --> "highcontrast", which however also switches from a dark to a light background, so might not be so pleasing for you... There probably IS a way to change just the small item you're interested in, but I guess only manually (ie changing files or such...) - and I have no idea how you'd go about that
Hope that helps a bit, regards