[Bug Report] incorrect dependencies for custom-theme

### Element UI version
2.12.0

### OS/Browsers version
Chrome Version 78.0.3904.87 (Official Build) (64-bit)

### Vue version
2.6.10

### Reproduction Link
https://github.com/attx/element-theme

### Steps to reproduce
Go through https://element.eleme.io/#/en-US/component/custom-theme

npm: 6.12.0
node: v12.13.0

1. npm i element-theme -g
2. npm i element-theme-chalk -D
3. et -i
4. error appears (primordials is not defined)

### What is Expected?
no errors

### What is actually happening?
Following error appears:

➜ theme et
fs.js:27
const { Math, Object } = primordials;
^

ReferenceError: primordials is not defined
at fs.js:27:26
at req_ (/Users/x/.nvm/versions/node/v12.13.0/lib/node_modules/element-theme/node_modules/natives/index.js:143:24)
at Object.req [as require] (/Users/x/.nvm/versions/node/v12.13.0/lib/node_modules/element-theme/node_modules/natives/index.js:55:10)
at Object. (/Users/x/.nvm/versions/node/v12.13.0/lib/node_modules/element-theme/node_modules/graceful-fs/fs.js:1:37)
at Module._compile (internal/modules/cjs/loader.js:956:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:973:10)
at Module.load (internal/modules/cjs/loader.js:812:32)
at Function.Module._load (internal/modules/cjs/loader.js:724:14)
at Module.require (internal/modules/cjs/loader.js:849:19)
at require (internal/modules/cjs/helpers.js:74:18)