The Santa Fe Institute[0] has some courses, tutorials, and publications[1] mainly, it seems, around complexity science. Complexity science isn't the same as systems theory/science, but most (all?) systems are complex so systems thinking usually necessitates some grasp of complexity. It's probably a good place to start.

They also have short courses on information theory, differential equations, and such.

[0] https://www.santafe.edu/ [1] https://www.santafe.edu/research/results/books