In answer to the original question: Very, very hard.
Theme isn't something decreed by staff at the beginning of the game. Theme is "living" and evolving. If someone doesn't enforce the theme, then that becomes your theme. Does player X drive through the city in invisible cars and nothing happens to him? Then your game's theme is now one in which you can drive through the city in invisible cars and nothing will happen to you.
The theme that the books set out is one in which "everything happens for a reason". There is a reason a sphere has a rule against X, Y, and Z. Because when you do X, Y, and Z - bad stuff happens. If I get into your game and do X, Y, and Z and nothing happens, then those rules mean less and less to me each time I get away with it. No different than RL, really. So if I'm breaking thematic rules ICly in private and no one finds out, that's fine, because no one found out, therefore no one knows to apply the consequences. But if anyone does find out and no consequences are applied, I'd be less and less concerned about following those rules. The same goes if I see other people breaking those rules and nothing happens.
Having the Masquerade in place wasn't just for the cool factor of being a vampire. It was because there would be consequences for all vampires if the world at large acknowledged the existence of vampires and therefore, vampires policed themselves harshly for breaking the Masquerade. As a result, the enforcement of the Masquerade led to many other rules/policies being in place, politically and socially, and was a major part of being a vampire in a civilized society. So if you lose a major pillar of your theme to non-enforcement, many other pillars collapse as well. For example, if breaking the Masquerade isn't important in this city, what does it matter if I make childer and set them loose without proper instruction? (This is a very, very elementary understanding of Vampire from many editions ago - but I think the point still gets across.)
I can't tell you the number of times there were reminders in nWoD Changeling that Changelings were supposed to be very secretive and shy away from public attention because of True Fae and Loyalists when the reality of the game was that there were beaches of glitter flooding the streets of the city from Glamour being thrown around with abandon in public and no one was ever, EVER carted off back to Arcadia from being tracked down by True Fae, Loyalists, or anything else. But when a PC was killed by another group of PCs and the promise of actual consequences was a reality, PCs got real secretive, real quick, and turtled up with friends for protection (exactly how the entire sphere should have been acting, according to the established theme) - except neither side of PCs were Loyalists, just feuding.
The supposed theme was one thing, the actual theme on the game was something else entirely. What it all came down to was consequences.
So theme enforcement should happen. How varies. You can go with direct staff intervention, NPC interdiction, or if you're brave enough - PC enforcement. But your theme will be what it will be, with or without you.