The feature of loading gradient palettes was removed (see also bug 120369 and bug 131043). So always only a file which is named 'standard.sog' (from <instdir>/share/palette or from user/config) can be used. But even if you rename the files 'classic.sog' or 'modern.sog' to 'standard.sog' you cannot use them, because they have wrong namespaces. So either do not install these files at all or repair their namespaces. If you will test it, you need to know, that a 'standard.sog' in the 'user' folder always has preference of the file 'standard.sog' in the installation folder. The defined gradients are not mixed. The default gradients you see, are not from the installation folder, but were copied to the 'standard.sog' in the 'user' folder when you first time add a gradient.