The BBCode for it appears to be [details] [/details]. Still I’m not a fan of the way the new spoilers look, I don’t if there is a way to change it, I doubt it.
Like everything here, the spoiler is defined by CSS style sheets which we have complete access to (which is a lot more than can be said for the old forum). This new weird blue box is the result of 10 minutes messing around. If you let me know how you’d prefer it or anything else to look, I can fulfill your wishes.
I just tried it on my phone and I can use it if I turn my phone sideways to add more width to the post editor. Of course if you remember the bbcode for it you can also add them that way.