The CSS files in [`/bootstrap/dist/css/`](https://github.com/twbs/bootstrap/tree/master/dist/css) are compiled from the [Sass](http://sass-lang.com/) source files in [`/bootstrap/scss/`](https://github.com/twbs/bootstrap/tree/master/scss) (or the [Less](http://lesscss.org/) source files in [`/bootstrap/less/`](https://github.com/twbs/bootstrap/tree/master/less)) via a [Grunt](http://gruntjs.com) task that invokes the [Sass preprocessor](http://sass-lang.com/) (or the [Less preprocessor](http://lesscss.org/)). 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/`](https://github.com/twbs/bootstrap/tree/master/scss) (or the `.less` source files in [`/bootstrap/less/`](https://github.com/twbs/bootstrap/tree/master/less)) instead.

Bootstrap isn't written directly in CSS, but is instead written in [Sass](http://sass-lang.com/) (or in [Less](http://lesscss.org/)), stylesheet languages that compile down to CSS.