Theme Switching

I’ve searched but couldn’t find a answer to my question.
What I need is this:

I want the viewer of the site to be able to choose between a dark site and a lite site. I already made 2 similar themes and uploaded it. One is dark style and the other is light style.

Now in the header I’d like to put the option:
Choose dark site (when it’s light)
Choose light site (when it’s dark)

An example of this is at https://partyflock.nl/