Non-live preview of themes and override stylesheet

Once upon time if you wanted to see how your portfolio would look with new theme you'd "enable" that theme and take a look at it. Problem was, the whole world would be looking at the same thing which meant that if you were also making changes to your override stylesheet, the whole world could see this work in progress too.

But fear ye not! Now you can preview a theme with your stylesheet and your projects and make whatever changes you like while your throngs of adoring fans view your other (active) theme. When you're done, you just "enable" that theme and the world can now see what you've been up to.