In case you haven't noticed yet: The background image of the current Steam sale shows a planet of sorts, and if you scroll all the way down, you see the other half of that planet. As logic demands, everything is upside down on the bottom - including the store menu!
See for yourself! There even is the search field, which displays the text in a mirrored fashion. If you open a game page brought up by the mirrored search field, that page loads an upside-down version of said page as well!