Bootstrap LESS code was split into different files. Variables are defined in .less variables, so you can edit them all in one place. To compile Bootstrap, you must compile the main bootstrap.less file. You cannot compile one file because these files depend on other files (basically .less and mixins.less variables). Starting with version 3.2. also mixes are divided into different files. mixins.less imports these mixins files.
, mixins :
@import "variables"
@import "mixins"