For what it's worth... I also was a bit annoyed in particular by the tomboy and pidgin icons. Too dark!
I changed them so that now Tomboy shows the lighter gray icon and Pidgin shows its original good old icons. It looks like this:
panelicons.png
I use Mint-X icons, and what I changed was, for Tomboy replace:
/usr/share/icons/Mint-X/status/22/tomboy-panel.png
/usr/share/icons/Mint-X/status/24/tomboy-panel.png
With:
/usr/share/icons/Mint-X-Dark/status/22/tomboy-panel.png
/usr/share/icons/Mint-X-Dark/status/24/tomboy-panel.png
And for Pidgin, I replaced all icons from:
/usr/share/pixmaps/pidgin/tray/Mint-X/status/16/*
/usr/share/pixmaps/pidgin/tray/Mint-X/status/22/*
/usr/share/pixmaps/pidgin/tray/Mint-X/status/32/*
/usr/share/pixmaps/pidgin/tray/Mint-X/status/48/*
With:
/usr/share/pixmaps/pidgin/tray/hicolor/16x16/status/*
/usr/share/pixmaps/pidgin/tray/hicolor/22x22/status/*
/usr/share/pixmaps/pidgin/tray/hicolor/32x32/status/*
/usr/share/pixmaps/pidgin/tray/hicolor/48x48/status/*
I had a hard time figuring this out and finding out that the panel icons for Pidgin are not in the /usr/share/icons/ folder where I was expecting them to be. This should be an easy way for anyone wanting to use Pidgin default icons.
If you're going to try this make sure you backup files before replacing them and all that story.