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This got me too. It's not in preferences, you have to use the hamburger menu thingy. Then, it's shoved randomly at the bottom.

The overall design is really a mess in 57, although browsing does seem faster.

You can also switch themes in the Add-ons Manager: about:addons

Pretty weird to put it there instead of Preferences.

Use the hamburger menu, select "Customize", and it's at the bottom of the screen.

Found by going to Tools->Add-Ons and then clicking Themes