How do you feel about sites having selectable themes?

I would like to ask your opinions about sites that have multiple themes available and allow the user to change the theme while navigating the site.

What do you think about this ?

Is it a cool or attractive option ?

Is it unprofessional ?

Is it worth the work of development ?

Have you implemented something like this ?

I have been thinking about developing a system for a site to allow support for multiple themes but I am not sure if that would be professional and worth it.