code: function exportCssFile(spriteTask) {
      var cssContentList = [],
          styleSheetArray = spriteTask.styleSheetArray,
          cssContent = '',
          compressOptions = spriteConfig.output.compress,
          fileName,
          fileName2, // 用于输出 log
          combinedCssRules,
          imageName,
          fileversion = spriteTask.fileversion;
  
      if (!styleSheetArray) {
          styleSheetArray = [spriteTask.styleSheet]
      }
  
      styleSheetArray.forEach(function(styleSheet) {
          cssContentList.push(styleSheetToString(styleSheet));
      });
  
      fileName = path.basename(spriteTask.cssFileName);
  
      fileName2 = path.join(spriteConfig.output.cssDist, fileName);
      fileName2 = path.normalize(fileName2);
  
      fileName = path.join(spriteConfig.workspace, fileName2);
      fileName = path.resolve(fileName);
  
      // 把合并了的样式统一在一起输出到文件的最后
      if (spriteConfig.output.combineCSSRule) {
          combinedCssRules = spriteTask.combinedCssRules;
          for (imageName in combinedCssRules) {
              var versionImageName = imageName;
              if (fileversion) {
                  versionImageName += '?' + fileversion;
              }
              cssContent += combinedCssRules[imageName].join(',') + '{' +
                  'background-image: url(' + versionImageName + ');' +
                  '}\n';
          }
      }
  
      cssContent = cssContentList.join('\n') + cssContent;
      if (compressOptions) { // 压缩
          if (!us.isObject(compressOptions)) {
              compressOptions = null;
          }
          cssContent = new CleanCSS(compressOptions).minify(cssContent);
      }
      nf.writeFileSync(fileName, cssContent, true);
      info('>>Output css:', fileName2);
  }
docs: **************************************************************** 7. 输出修改后的样式表 **************************************************************** 
  输出修改后的样式表
  @param  {SpriteTask} spriteTask