How does one become a theoretician? What does it mean to be an opening theoretician, and how does one achieve this status? Does it imply that one should not be an active tournament player? Apart from opening theoreticians are there middlegame and endgame theoreticians, too? Why are most theoreticians from eastern europe?
A theoretician is someone who has created a new opening, middlegame and / or endgame variation. You don't have to be a grandmaster to become one, and not all grandmasters are ones.

It may not apply if your variation is not tactical or is useless.

I don't think it matters where a theoretician comes from, as everyone has different chess experience, and whether you are an active tournament player or not.

People who have written about chess theory in general are chess theoreticians.