I can't change the Nebular theme

### Issue type

**I'm submitting a ...** (check one with "x")

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
I can't change the theme from `default` to any other theme. The theming completely disappear as well as the modules.
The Nebular theme works as expected only when using the `default` theme.

With default theme:

With dark theme (the same goes with corporate and cosmic themes):

It seems that the Nebular themes and modules are not working.

**Expected behavior:**
Well, the dark, cosmic or corporate themes should be displayed.

**Steps to reproduce:**
```
ng add @nebular/theme
? Which Nebular theme do you want to use: default
? Use customizable scss themes? Yes
? Set up browser animations for Nebular? Yes
```

**Related code:**
This works perfectly:
```ts
imports: [
// some imports...
NbThemeModule.forRoot({ name: "default" }),
// other Nebular imports...
],
```

This doesn't work at all:
```ts
imports: [
// some imports...
NbThemeModule.forRoot({ name: "dark" }), // the same goes with cosmic and corporate, only default works
// other Nebular imports...
],
```

### Other information:

**npm, node, OS, Browser**
Node: 12.19.0
npm: 6.14.8
OS: Windows 10
Browser: Mozilla Firefox and Google Chrome

**Angular, Nebular**
Angular: 11.1.4
Nebular: 7.0.0