In the Steam beta branch, you can now download a special v1.07 release of Fleshcult that has the first phase of mod support.
Steam Workshop isn’t supported just yet, so it’s not very interesting if you just want to play mods, but if you’re interested in creating them there’s new stuff to tinker with. So far it supports HTML/CSS mods for changing the way the game is presented. The ability to change the gameplay is something I’ll be working on next year.
To learn more about creating mods, you can consult the new Fleshcult Wiki which has a modicum of technical documentation. Feel free to add stuff I’ve glossed over. You’re also welcome to add info about playing the base game if you like. Happy holidays!