Seasons only modifies a few files, the shader files relating to map effects, the dynamic_masks code file and the ones related to it, this is so that winter works correctly.
The 00_graphics defines file, for snow purposes, and the 00_defines file. These files are not strictly needed, however they do make the snow a bit more immersive and make things run a bit smoother.
child_birth_on_actions is modified, but only to add : "season_flavor_events.00015" to line 386, so that children can get seasonal nicknames when applicable.