Dropbox systray-icons shows a white ugly background which seems to be a never-ending behavior in GNU/Linux. I found an ugly work-around around this by first creating a 32x32 pixel large png-image with a background matching the panel background of your systray. Then, copy-paste your custom Dropbox-icon in your created image. Then overwrite the files located at:
and perhaps in
depending of which version you use (I use Dropbox 3.4.4).
Here are my result matching the icons to my Laponia Light theme (xexpanderx.deviantart.com/art/…
PS: Let me now if you have a better solution.