The button is at the bottom of the screen to the right. Visit an example page [1], and it should be there at the very bottom right corner. It is set to only appear on wide screens (1600px+), where the "wasted space" can actually be utilized.

Seems being directly linked to pages doesn't show the new design sometimes if you aren't logged in. But I'll provide the page where they talk about the new theme itself and the expand button [2].

[1] https://en.wikipedia.org/wiki/2023_Antiguan_general_election

[2] https://en.wikipedia.org/wiki/Wikipedia:Vector_2022#How_to_e...