Demo dashboard for wordpress and color customization


I want to sell my theme and it would be nice if my customers can see how the dashboard works. I'm using the ag custom admin plugin to limit the accesses for my customers.
Now, I am looking for a plugin to let potential customers acces the dashboard via a link(without login) and let them browse trough it without letting them upload or change anything about the current website.
I saw that this is possible, but I forgot where I saw this.

Further, I always see on the theme forest themes that they have a side screen where potential customers can customize the theme colors. How do they do that?

Thank you in advance.

Kind regards,