Angular prod build fails when $nb-enable-css-custom-properties and inlineCritical setting is true

### Issue type

**I'm submitting a ...**

* [X] bug report
* [ ] feature request

### Issue description

**Current behavior:**
The build fails with the error message: `✖ Index html generation failed.
undefined:16:372306: property missing ':'`

**Expected behavior:**
The build should succeed

**Steps to reproduce:**
1. Create a new Angular 11.2 project
2. Set $nb-enable-css-custom-properties to true
2. Enable inlineCritical optimization in angular.json
3. Run a production build with `ng build --prod`
4. The error occurs after `Copying assets complete` message

The error does not occur when you either disable inlineCritical optimization or set $nb-enable-css-custom-properties to false

### Other information:

**npm, node, OS**
```
Node: 12.16.2
npm: 6.14.4
OS: macOS Big Sur 11.2.1
```

**Angular, Nebular**
```
angular: 11.2.0
nebular: 7.0.0
```