Browser back button not working as expected in Add New Themes

[melchoyce]

When browsing through "add theme" previews, my browser (Chrome) back button doesn't work as expected. If you preview a theme, close out, preview another theme, then click the "back" button in your browser, the URL shows as the previously theme you previewed. Nothing on the screen changes.

If you preview another theme, close out, and preview another one after that, click back on your browser, and close out of the preview, you're shown the first theme you look at as a search result.

See it in action: ​https://cloudup.com/cq9nf4SgHkm

[iseulde]

Related: #28734?

[obenland]

Duplicate of #28734.