index.js

var loaderUtils = require('loader-utils');
var path = require('path');
var fs = require('fs');

var bulmaPattern = /node_modules\/bulma\/.+sass$/;

module.exports = function (source) {
  if (this.cacheable) this.cacheable();
  var callback  = this.async();
  
  // Only act on bulma sass imports
  if(!bulmaPattern.test(this.resourcePath)) {
    return callback(null, source);
  }
  
  var config = loaderUtils.getLoaderConfig(this, 'bulmaLoader');
  var themeName = config.theme;
  
  if (!themeName) {
    return callback(new Error('No `theme` specified in the bulma-loader options! See: https://github.com/stipsan/bulma-loader#usage'))
  }
  
  var themePath = path.resolve(themeName);

  var themeExists = fs.existsSync(themePath);
  if (!themeExists) {
    return callback(new Error(`The path ${themePath} does not exist!`))
  }

  this.addDependency(themePath);

  fs.readFile(themePath, "utf-8", function(err, theme) {
    if(err) return callback(err);
    callback(null, theme + '\n' + source);
  });
};