Questions and possible solutions:
1) Q: how can we add and remove a colour scheme to a set?
A: we could add remove a colour scheme to a set and share it with
other local users modifying a file into /usr/share/colorschemes/
directory (i.e. /usr/share/colorschemes/John or
/usr/share/colorschemes/mycustom etc).
2) Q: how can we set the current colour scheme?
A: we could sets the current colour scheme selecting a set and a
colour scheme using a GUI and changing the relative key in gconf when apply.
3) Q: how can we install or uninstall a colour schemes set?
A: we could install or uninstall a colour schemes set copying or
deleting the file into the /usr/share/colorschemes/ directory, and share
it with other local users.
5) Q: how can we export a colour schemes set for world wide sharing?
A: we could export a colour schemes set simply copying the file from
/usr/share/colorschemes/ directory.
6) Q: how can a themer add a default colour scheme set for his theme?
A: a themer could add his default colour scheme for his theme
installing a file into /usr/share/colorschemes/ directory.
The idea of colour schemes stored in various custom sets (files) is born
generalizing the need of colour schemes set by themes (as requested). I
am sure that this is only a possible solution and there are many others
even more valid. I will continue searching one we can consider "the best"