The CSS files in /bootstrap/dist/css/ are compiled from the Sass source files in /bootstrap/scss/ (or the Less source files in /bootstrap/less/) via a Grunt task that invokes the Sass preprocessor (or the Less preprocessor). These compiled CSS files should not be edited manually when developing Bootstrap itself. Any such edits will be overwritten and lost the next time the Grunt task runs.
You should edit the .scss source files in /bootstrap/scss/ (or the .less source files in /bootstrap/less/) instead.
Bootstrap isn't written directly in CSS, but is instead written in Sass (or in Less), stylesheet languages that compile down to CSS.