I thought Bootstrap is about frontend being backend-agnostic or what do I miss?
I think you have some things mixed up, Bootstrap is a front-end layout framework built with a CSS preprocessor, in this case SASS. It's got nothing to do with backend.
You are asking if you can use a different preprocessor, for which the answer is 'no'.
I think the answer is "yes" for those willing to use a fork. If your favorite preprocessor is popular I bet someone already made one.
It was just an example that it won't work, sorry for the confusion.