Currently there is support for something akin to what you are describing on the server-side.
If you have the latest standalone client, you can see this in action by installing and enabling the net.lugdunon.world.tupelo.tui mod that resides on the official artifactory (more detail on this here:
http://www.indiedb.com/games/lugdunon/news/beta-064-released).
That will swap out the tileset, ui, and icon assets with the alternate style that is currently being developed.
You can also do the same with the sprite assets as they are in a separate mod, but we haven't yet begun to create the alternate implementations for them yet.
As far as the ability to override on the client-side with your preferred tileset / sprites: this is something that I have not given a whole lot of thought to, however I can see the merit in allowing something like this. I'll log this as a feature request for future reference.
Cheers,
Chris