Shaders
Generally many in the Theatrical like to specifically use one particular mod called Shimmer, which adds colored lighting to Minecraft - and Theatrical has support for it!
However, generally shaders aren't very compatible with Shimmer!
Please see their wiki here for extra info, and an example of adding support for BSL shaders.
There is a support thread in the Theatrical Discord server here where you can find others working with getting Shimmer and shaders working together.