These icons are the remake, based on the icon, taken here: http://gnome-look.org/content/show.php?content=126759. Ubuntu icons from this set aren't very impressive, as they fade with the panels, but Elementary icons are pretty good, though they have another color scheme. So I've taken an empty pot icon, made some Inkscape and GIMP hacking and voila.
Caffeine icons for the dark theme are located in "/usr/share/icons/ubuntu-mono-dark/status/22/" and for the light theme - in "/usr/share/icons/hicolor/22x22/status/". Their names are "caffeine-cup-empty.png" and "caffeine-cup-full.png". You can make a backup before the installation.
Extract files from the archive. Use the icons from the Dark folder if you use Ambiance + ubuntu-mono-dark icon set and the icons from the Light folder if you use Radiance + ubuntu-mono-light icon set. Overwrite the existing icons in the respective folders I mentioned earlier. You will need "sudo" to do that.
Relogin to see the changes.