The broccoli-postcss-single plugin runs your css through postcss plugins of your choosing.

npm install --save-dev broccoli-postcss-single

var compileCSS = require('broccoli-postcss');

var outputTree = compileCSS(inputTrees, inputFile, outputFile, plugins, map);

• inputTrees: An array of trees that specify the directories used by Broccoli. If you have a single tree, pass [tree].
• inputFile: Relative path of the main CSS file to process.
• outputFile Relative path of the output CSS file.
• plugins An array of plugin objects to be used by Postcss (a minimum of 1 plugin is required). The supported object format is module: the plugin module itself, and options: an object of supported options for the given plugin.
• map An object of options to describe how Postcss should handle source maps.

/* Brocfile.js */
var compileCSS = require('broccoli-postcss');
var cssnext = require('cssnext');

var plugins = [
    {
        module: cssnext,
        options: {
            browsers: ['last 2 version']
        }
    },
];

var outputTree = compileCSS(['styles'], 'app.css', 'app.css', plugins, map);
module.exports = outputTree;