IPS 5 doesn't support custom themes anymore.
Just the ability to slightly alter the theme by adding custom/additional js/css and that's it.
Developers of that software made it so that every forum that uses IPS 5 is some what unified instead of every forum has their own theme.
That's why also you don't see a marketplace anymore for IPS, they shut it off.
One of the reasons why a lot people switched off this software and moved on to other alternatives, such as XenForo etc.