Update doc of theme config [5.0]

[RammusXu]

https://blog.skk.moe/post/hexo-5/#%E9%80%9A%E8%BF%87-npm-%E5%AE%89%E8%A3%85%E4%B8%BB%E9%A2%98

This part is pretty useful. Should be put in https://hexo.io/zh-tw/docs/themes.html

[SukkaW]

So far we are still working on some rare issues about the feature (e.g. the npm package under owner likes @user/hexo-theme-custom). After all issues being fixed then the Hexo official documents could bring up the detailed instruction.

[RammusXu]

Thanks for replying. Is it ok to keep this issue open?

[SukkaW]

@RammusXu Sure. The issue is now labeled as documentation.