There are a few of us that teach theatre style playing, so lessons - live or on line - are the best way to go.
Tom Horton has done some great videos and publications on this style of playing. And remember that there are different styles of playing within 'theatre style'. 'Blackpool' is the one that many people think of. 'Old style British', like Sydney Torch, Robinson Cleaver etc, is another. The lush 'romantic theatre' style, like Jackie Brown, or the 'American' style, like George Wright, Buddy Cole, Billy Nalle, Ashley Miller and many others. There are a few books from the USA, look for ones by William McMains. Not easy at all, but worth the effort.
And you need an organ capable of creating theatre sounds, or use the organ to drive a virtual theatre organ.