Let user try various theme colors in demo? (client insists)

Please think of this as a technical exercise, not a question of whether it should be done. I mean, no bunnies are going to be harmed, just a bit of extra work made, right? And that’s good, because they are very cute bunnies. With kittens riding them. Chased by ducklings.

I’m colorblind and making a website for a client. The person who has the most interest in choosing colors is not someone who will be given any access to the admin view. They need to be able to see the colors applied to the site so they can see why some work (and some really don’t!). Furthermore, they want to do so on the fly and not via edits on my part (on the phone, or iteratively via email).

I need to give them a way for a reader to edit the color scheme, preferably via a color chooser, or just pasting in hex codes. This is not going to be on the final live site but is on the user end, not admin area.

I’ve found nothing in searching for something along these lines so I would appreciate any help, be it code snippets or generalized outlines of how to approach it, or even just some thoughts on how to do so.

Thank you!