function toCSS(options, filePath, callback) {
        options = options || {};
        var str = fs.readFileSync(filePath, 'utf8'), addPath = path.dirname(filePath);
        if (typeof options.paths !== 'string') {
            options.paths = options.paths || [];
            if (!contains(options.paths, addPath)) {
                options.paths.push(addPath);
            }
        } else {
            options.paths = [options.paths]
        }
        options.paths = options.paths.map(searchPath => {
            return path.resolve(lessFolder, searchPath)
        })
        options.filename = path.resolve(process.cwd(), filePath);
        options.optimization = options.optimization || 0;

        if (options.globalVars) {
            options.globalVars = options.getVars(filePath);
        } else if (options.modifyVars) {
            options.modifyVars = options.getVars(filePath);
        }
        if (options.plugin) {
            var Plugin = require(path.resolve(process.cwd(), options.plugin));
            options.plugins = [Plugin];
        }
        less.render(str, options, callback);
    }
@param {Object} options
@param {string} filePath
@param {Function} callback