They don't work because the lavender theme seems to have some overwrites to fix 100% not fitting in some browsers:
https://github.com/NodeBB/nodebb-theme-lavender/blob/master/less/style.less#L80-L90
Because of the .home ... it's more specific and overwrites the default bootstrap classes.
To make your own ones working you have to use sth. like this for now:
@media all and (min-width: 2048px) {
.home .col-xl { width:8.33333% }
}
ping @psychobunny maybe he'll fix that for standard bootstrap classes (would need to include all col-md and col-lg classes definitions into theme thought). 🎁
PS: you can see/change the bootstrap settings in lavender here, so it's lg: 1200px+, md: 992px+, sm: 768px+, xs: 480px+
But beware that this affects all columns on each page not only on home... If you intend to only change home you should rather overwrite them with .home .col-... as in above code-block 😉
