PopMan

Is it possible to make the toolbar's background transparent? If so, how?

I thought I was able to before but not anymore

The background color is simply the color of the left most pixel at the bottom of the bitmap. All pixels that have the same color will automatically show the background color of the toolbar. Note that there may be problems depending on the color depth. You might have to try out until you find a suitable depth.

Btw, I tested your Gnome theme and it looks nice. There seems to be no background color problem.