Gnome-panel requires icons with the size of 22x22px, e.g. ooo-icons are only available in 24x24px size. Human-Theme is doing the following: it is adding a "gtk-icon-sizes = "panel-menu=24,24"" entry in gtkrc, so scaling is not necessary. When using Clearlooks-theme, the "hack" is not applied by default, therefore all icons that are not available in 22x22px-size are looking blurry. You can prevent this by creating a ~/.gtkrc-2.0 file with the "gtk-icon-sizes = "panel-menu=24,24"" entry in it (or "hacking" the system-wide gtkrc in the /usr/share/themes/YOURTHEME/gtk-2.0). This should work, at least it works for me.