I personally believe the easiest way to accomplish this is by duplicating your dashboard, making the color changes you like, then link the two via a dashboard navigation button. It is so simple to do...not complicated at all...and you have all the flexibility in the world.
I recently wrote about this in the blog post linked below. The blog walks through an example of using this technique to build collapsible menus, but at the bottom there is a list of all kinds of use cases, including color theming. As a side note, this blog post includes a link to my brother's blog that discusses an alternative way to do color themes.
Let me know if this works out for you!