postcss-loader config information

Nobody has claimed this yet.

I have a postcss.config.js file like this:
```
module.exports = {
plugins: {
'postcss-preset-env': { },
},
};
```
What would be the best way to include `post-css-variables`? As a plugin of postcss or postcss-preset-env

Either way i´m having this error:
```
ERROR in ./node_modules/framework7/css/framework7.bundle.css (./node_modules/css-loader/dist/cjs.js!./node_modules/postcss-loader/src!./node_modules/framework7/css/framework7.bundle.css)
Module build failed (from ./node_modules/postcss-loader/src/index.js):
TypeError: Cannot read property 'indexOf' of undefined
at path\to\project\node_modules\framework7\css\framework7.bundle.css:2813:3
at path\to\project\node_modules\postcss-initial\index.js:20:22
at path\to\project\node_modules\postcss\lib\container.js:190:18
at path\to\project\node_modules\postcss\lib\container.js:135:18
at Rule.each (path\to\project\node_modules\postcss\lib\container.js:101:16)
at Rule.walk (path\to\project\node_modules\postcss\lib\container.js:131:17)
at path\to\project\node_modules\postcss\lib\container.js:148:24
at AtRule.each (path\to\project\node_modules\postcss\lib\container.js:101:16)
at AtRule.walk (path\to\project\node_modules\postcss\lib\container.js:131:17)
at path\to\project\node_modules\postcss\lib\container.js:148:24
at Root.each (path\to\project\node_modules\postcss\lib\container.js:101:16)
at Root.walk (path\to\project\node_modules\postcss\lib\container.js:131:17)
at Root.walkDecls (path\to\project\node_modules\postcss\lib\container.js:188:19)
at Object.plugin (path\to\project\node_modules\postcss-initial\index.js:19:9)
at promise.then (path\to\project\node_modules\postcss-preset-env\index.js:443:97)
@ .\node_modules\framework7\css\framework7.bundle.css 2:26-125 22:4-35:5 25:25-124
@ .\src\js\app.js
@ multi .\src\js\app.js
```
I want to discard that this is a postcss-loader or webpack problem

Start with the postcss.config.js shown in the report and trace the postcss-preset-env and postcss-initial entries in the provided stack trace. Reproduce the webpack build against framework7.bundle.css, then determine whether plugin placement or the reported undefined value causes the failure.

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