使用webpack raw 方式导入文件，防止直接渲染，通过动态添加样式到<style>实现主题样式切换。
// 导入样式文件
import broadway from '!raw!./style/dhtmlxgantt_broadway.css'
import meadow from '!raw!./style/dhtmlxgantt_meadow.css'
import skyblue from '!raw!./style/dhtmlxgantt_skyblue.css'
import terrace from '!raw!./style/dhtmlxgantt_terrace.css'
...
themes:[terrace,broadway,meadow,skyblue],
...
document.getElementById('style-container').innerHTML = this.themes[0];
...
Github: https://github.com/yansenlei
