just get "ERROR Asset render failed: directory/*.css".

Because I want to write my own hexo-theme and want to use hexo-renderer-less to render less to css. But the official README file is so poor introduce that I don't konw how to use it to render my less file to css file so that the theme will show normal.(I have install less use npm before) .
The question list is below:
1 Please tell me how should I do to use it? in the "_theme config.yml" should point out a entry less file or should list all the less file so hexo-renderer-less can render them correct?
less:
paths:
- themes/graces/source/less/main.less
2 in the hexo-renderer-less directory 'index.js' file , wen I use "hexo g" the common line console "TypeError: Cannot read property 'toCSS' of undefined". Seems something wrong about my theme config ?
Waiting for you answer,Thx!