Changed theme should be applied to all pages in a session

[rkiesler1]

To reproduce:

1. From the home page, open any other page (e.g., docs) in a new browser tab
2. Change the theme on the home page
3. Refresh the secondary page in the separate tab

I expected the new theme to be applied, but the page still renders using the original them

[cferdinandi]

It's really only intended as a quick preview, but... I like this idea, so I'll flag it as a feature request. Should be easy enough to do with sessionStorage and a quick update to the code.