[Bug Report] 合并table的表头后，主题色设置的表头背景色不正确

### Element UI version
2.13.0

### OS/Browsers version
chrome

### Vue version
3.0.0-alpha.9

### Reproduction Link
https://codepen.io/pen?__cf_chl_jschl_tk__=9a3487b68475d73bbf0357aa0ced3cdf3dbdc2aa-1584509554-0-AVsQgdpmGDkWkWKBQNs8ghz2t_jtkpvmHHnEdO-Lml7f0wOHcsvgwCulLutJXJClfv9rTuAZnNyR8Fa5pud4aY_nSi3uYD2NwVvYfgi7yB-4fa_zJ4oR3E91J94Z3_vGQZDGjaKwvJNx3QutyUjRcxSn8TPzRxe4xE31Bpt7ewBGDa-3RXAA3jOuhO0V2z0ZifotTFJ7DfLnhA5aaPBInQll--E-zKPBvNm1NQKCRxAdrgyfl6nsvg6DLltTKS3GsatMGKIv6ZwOJYgcVHs2gQiiXVAH4Mc9mc01rRenLN_8agfC_GQMzx3sGPIMEtT6GL4yme5t9rTVMyoIfOCixLw

### Steps to reproduce
\element-ui\packages\theme-chalk\src\table.scss

&.is-group {
th {
background: $--background-color-base;
}
}

这个应该是background: $--header-background-color

### What is Expected?
这个应该是background: $--header-background-color

### What is actually happening?
background: $--background-color-base;