Pages within Kartris have a default masterpage set in the block right at the top, for example the Product.aspx page which displays a product has this at the top:

If this is unchanged, then Kartris will enforce the various skin rules to determine which skin to use. This means looking at the Skin.config file (if present) and the choice of skin set in the Kartris back end. However, if this code is changed to point to another .master file, Kartris will use that instead and not run the various rules. So if you want to set all products to use a different .master page, you can edit this code at the top of Product.aspx to achieve this.