Compilation of Nebular theme is too slow

### Issue type

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

* [X] bug report
* [ ] feature request

### Issue description

I'm recently upgraded from Nebular v. 3.5 > v.4.4 and Angular v. 7.4 > 8.2. The migration went well, and everything is currently sat up and working. The problem I'm experiences has too to do with the compilation time of the Nebular theme(s). When adding it, the compiler stops for about 10-20 seconds before continuing, a time-span that reduces productivity across the team.


**Expected behavior:**
The compilation time of the Nebular theme(s) should be more or less fluent.

**Steps to reproduce:**
The reproduction of the problem has the minimum requirement for a Nebular application with two themes. When cloning the repository and running `npm start`, you will see the compilation of the application is stopping when it hits the `styles.scss` file.

You can view the entire application setup at the [following ](https://github.com/mathisGarberg/nebular-theme-issue)GitHub repository.

**Other information:**
```
-------------------------------
"@angular/animations": "~8.2.7",
"@angular/cdk": "^8.0.0",
"@angular/common": "~8.2.7",
"@angular/compiler": "~8.2.7",
"@angular/core": "~8.2.7",
"@angular/forms": "~8.2.7",
"@angular/platform-browser": "~8.2.7",
"@angular/platform-browser-dynamic": "~8.2.7",
"@angular/router": "~8.2.7",
"@nebular/eva-icons": "4.4.0",
"@nebular/theme": "^4.4.0",
"@nebular/date-fns": "4.3.1",
```
**Note:**
If you want me to provide any further information, please let me know.