Why LESS?

Bootstrap is made with LESS at it's core, a dynamic stylesheet language created by Alexis Sellier. It makes developing systems-based CSS faster, easier, and more fun.

What's included?

As an extension of CSS, LESS includes variables, mixins for reusable snippets of code, operations for simple math, nesting, and even color functions.

Learn more

Visit the official website at http://lesscss.org to learn more.

Variables

Managing colors and pixel values in CSS can be a bit of a pain, usually full of copy and paste. Not with LESS though—assign colors or pixel values as variables and change them once.

Mixins

Those three border-radius declarations you need to make in regular ol' CSS? Now they're down to one line with the help of mixins, snippets of code you can reuse anywhere.

Operations

Make your grid, leading, and more super flexible by doing the math on the fly with operations. Multiple, divide, add, and subtract your way to CSS sanity.

Grayscale colors

Accent colors

Grid system

Typography

Buttons

Forms

Navbar

Form states and alerts

Bootstrap was built with Preboot, an open-source pack of mixins and variables to be used in conjunction with Less, a CSS preprocessor for faster and easier web development.

Check out how we used Preboot in Bootstrap and how you can make use of it should you choose to run Less on your next project.

How to use it

Use this option to make full use of Bootstrap’s Less variables, mixins, and nesting in CSS via javascript in your browser.

<link rel="stylesheet/less" href="less/bootstrap.less" media="all" />
<script src="js/less-1.1.3.min.js"></script>

Not feeling the .js solution? Try the Less Mac app or use Node.js to compile when you deploy your code.

What’s included

Here are some of the highlights of what’s included in Twitter Bootstrap as part of Bootstrap. Head over to the Bootstrap website or Github project page to download and learn more.

Variables

Variables in Less are perfect for maintaining and updating your CSS headache free. When you want to change a color value or a frequently used value, update it in one spot and you’re set.

// Links
@linkColor:         #8b59c2;
@linkColorHover:    darken(@linkColor, 10);

// Grays
@black:             #000;
@grayDark:          lighten(@black, 25%);
@gray:              lighten(@black, 50%);
@grayLight:         lighten(@black, 70%);
@grayLighter:       lighten(@black, 90%);
@white:             #fff;

// Accent Colors
@blue:              #08b5fb;
@green:             #46a546;
@red:               #9d261d;
@yellow:            #ffc40d;
@orange:            #f89406;
@pink:              #c3325f;
@purple:            #7a43b6;

// Baseline grid
@basefont:          13px;
@baseline:          18px;

Commenting

Less also provides another style of commenting in addition to CSS’s normal /* ... */ syntax.

// This is a comment
/* This is also a comment */

Mixins up the wazoo

Mixins are basically includes or partials for CSS, allowing you to combine a block of code into one. They’re great for vendor prefixed properties like box-shadow, cross-browser gradients, font stacks, and more. Below is a sample of the mixins that are included with Bootstrap.

Font stacks

#font {
  .shorthand(@weight: normal, @size: 14px, @lineHeight: 20px) {
    font-size: @size;
    font-weight: @weight;
    line-height: @lineHeight;
  }
  .sans-serif(@weight: normal, @size: 14px, @lineHeight: 20px) {
    font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
    font-size: @size;
    font-weight: @weight;
    line-height: @lineHeight;
  }
  ...
}

Gradients

#gradient {
  ...
  .vertical (@startColor: #555, @endColor: #333) {
    background-color: @endColor;
    background-repeat: repeat-x;
    background-image: -khtml-gradient(linear, left top, left bottom, from(@startColor), to(@endColor)); // Konqueror
    background-image: -moz-linear-gradient(@startColor, @endColor); // FF 3.6+
    background-image: -ms-linear-gradient(@startColor, @endColor); // IE10
    background-image: -webkit-gradient(linear, left top, left bottom, color-stop(0%, @startColor), color-stop(100%, @endColor)); // Safari 4+, Chrome 2+
    background-image: -webkit-linear-gradient(@startColor, @endColor); // Safari 5.1+, Chrome 10+
    background-image: -o-linear-gradient(@startColor, @endColor); // Opera 11.10
    background-image: linear-gradient(@startColor, @endColor); // The standard
  }
  ...
}

Operations

Get fancy and perform some math to generate flexible and powerful mixins like the one below.

// Griditude
@gridColumns:       16;
@gridColumnWidth:   40px;
@gridGutterWidth:   20px;
@siteWidth:         (@gridColumns * @gridColumnWidth) + (@gridGutterWidth * (@gridColumns - 1));

// Make some columns
.columns(@columnSpan: 1) {
  width: (@gridColumnWidth * @columnSpan) + (@gridGutterWidth * (@columnSpan - 1));
}

Compiling Less

After modifying the .less files in /lib/, you'll need to recompile them in order to regenerate the bootstrap-*.*.*.css and bootstrap-*.*.*.min.css files. If you're submitting a pull request to GitHub, you must always recompile.

Ways to compile

$ npm install less

<link rel="stylesheet/less" href="/path/to/bootstrap.less">
<script src="/path/to/less.js"></script>

$ lessc ./lib/bootstrap.less > bootstrap.css