The spellings "centre" and "theatre" are often used in the U.S. as an affectation, to make the buildings and institutions they represent seem a little more high class. Although my very favorite example of this lunacy is in Los Angeles: The L.A. Theatre Center.
Considering that there are more speakers of English in India than in America, we are outnumbered, so it's not worth complaining.
It is actually we Americans who are out of step. Noah Webster was the first American scholar to put together "a dictionary of the American language," and he deliberately altered the spelling of several very common inflections, just to make American writing look different from British writing. A strange manifestation of national pride.
So for the past century and a half we've had center, inflection, labor, realize and traveler, instead of centre, inflexion, labour, realise and traveller, spellings that had been established for half a millennium, most of them taken directly from the original French words.
It's no big deal. If you read enough British writing you'll stop noticing it. And nobody's ever going to make you start spelling that way unless you have to emigrate. And if you do, I promise you that spelling will be the very least important thing you miss about America.
