EDIT: Today I was pointed in the direction of a recently-developed app called "gtk-theme-config", which is available in this PPA. It was designed for Gnome 3, apparently, but works in XFCE as well. It shows up in the XFCE menu under Settings > GTK theme preferences. I took a screenshot:

As you can see, I've turned the setting for adjusting the the panel text on, and changed the color to white. It works; as you can see in the last screenshot I took, the text in the panel is indeed white now.
I still don't know where/how to do this manually, but the app gave me the desired result so I'm marking this as solved.


