All mojoPortal skins have a theme.skin file, those instructions are just for updating older skins that were made in artisteer 3 that now need to have different settings in theme.skin to be compatible with the latest mojoPortal.
So if you are using a skin that was created in Artisteer 3 then you need to replace the theme.skin file in the skin folder with one of the updated ones from some other artiteer 3 skin.
Skin files used by the site are located at /Data/Sites/[SiteID]/skins/[skinname]
where SiteID is usually 1 unless it is a multi site installation and skinname corresponds to skin name in the skin dropdown in
Administration > Site Settings
Note that theme.skin files for any specific version of Artisteer are the same (across skins made with that version) but they are different for different versions of Artisteer, so you need to make sure you know what version of Artisteer your skin was made with. Typically for the skins we ship we have a version indicator in the name of the skin