Actually, being a theatre background, it was easy to get into film lighting. They generally just want a 3 point system, a soft box or bounce from one side, flood the other, and something from behind to stop floating head syndrome. Film is also generally white, white, and more white. You also don't use nearly as saturated as colors in film as you do theatre since cameras aren't as smart as the human eye.
Theatre, the angles are often much more crazy: foot lights, shin/knee buster sides, side light, head side light, high side light, overhead, front, torm/box boom, back light, cyc/back drop light, and then all the special effect lighting, like gobos and specials. And then the color depth and theory is a whole ball of wax unto itself. Colors can go from nothing to so saturated, I have to replace gels during intermissions because it only lasts a couple minutes.
Its a good thing I enjoyed math in school, because I use a lot of algebra, geometry, and trigonometry as well as some physics just about every day. I am also a very visual person, which helps. I can just see stuff in my mind and how it should look before I do it. My point there, I guess, is film is basically the same thing every time, where as theatre will change depending on the show and what you are going for.