You will also need to recompile the source code with your custom formula. In main.cpp there's a comment "//All fractal equations" and under it you can add yours or modify an existing one depending on what you did.
CodeParade
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Yedoma Globula has similar gameplay to what you're describing, so you might what to check it out. Cheers! https://bananaft.itch.io/yedomaglobula
I have not written any papers on the collision detection used in the game (my main contribution here). However, there are many papers and tutorials already about ray marching rendering and procedural fractals or terrains. Specifically, you could check out the work of Inigo Quilez. But for collisions, I do have a more robust implementation in my python framework PySpace that generalizes the NP function to other SDFs besides Marble Marcher.
I'm not actively developing Marble Marcher right now, but the community edition is, that would be a better place to request new features: https://michaelmoroz.itch.io/mmce
Can you confirm if making this change to assets/vert.glsl fixes the problem for you? https://github.com/HackerPoet/MarbleMarcher/pull/37/files