If you examine the icon set in /usr/share/icons you'll find that within each icon set folder there are folders with the same names and within those folders are icon groups by size and within those folders icons for the same purpose have the same name. That means you can substitute icons from other themes that you prefer. I believe the icons your interested in are in the status folder. Some icon sets are more complete than others and some icons are used under multiple names by using links to point to a single image. Within folders like status the sub folders are by size with the svg folder containing scale-able icons. Once you've made changes to these icons you'll need to restart. I'm unsure if logging out and in is enough or if restarting unity will do so I just restart the system.