The VOIDRUNNER'S CODEX is coming! Explore new worlds, fight oppressive empires, fend off fearsome aliens, and wield deadly psionics with this comprehensive boxed set expansion for 5E and A5E!
Nope. You just slap on a theme and that's it. As with any monster tweaking you may want to make adjustments of some kind like remove or modify other powers in a few cases. Most themes don't add enough oomph to a monster to really make it much tougher, at least in theory.
It stays the same level, role, group role, and xp value.
However if it gets much more powerful I often treat it as 1 or 2 levels higher. If it gains powers that heal or buff it's allies I give it the leader secondary role.