[lubuntu] (gtk theming) Change coloring of lxpanel workspace switcher only?

How to change the coloring of the lxpanel workspace switcher, but *only* the workspace switcher? I'm using a slightly modified version of the Greybird theme.

Some screenshots - selected workspace is on the left, unselected on the right.

This is what it looks like: https://postimg.org/image/4uyneke3j/
On my monitor that's really hard to distinguish, so I'd like to make it look more like this: https://postimg.org/image/4gxbeytzz/

(I see some code for re-coloring XFCE's workspace switcher, but that code doesn't seem to affect the lxpanel workspace switcher...)