I thank you for the information but my question wasn't answered. I should have been clear when I asked my question. I am using an icon set called Still-Life and it is missing some mimetype images associated with ".glade", ".c", ".h", ".cpp", and ".css" files. I copied the mimetype images from another icon set, called OSX, into the Still-Life icon set and the ".css" and ".h" files now have images associated with those files, but the others do not. If I use the OSX icon set all the mimetypes have images associated with them. This is what is confusing me.
I have looked inside the theme.index files of the icon sets to see if I can figure out why the same mimetype images work when using one icon set, but not work for another.
To change my themes and icon sets I am using the program, lxappearance, if that makes a difference.