Custom CSS properties doesn't override global nebular styles

### Issue type

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

* [X ] bug report
* [ ] feature request

### Issue description

**Current behavior:**
after register a custom css property on a theme, it still shows the global style not overring it

**Expected behavior:**
it should override the global style

**Steps to reproduce:**
register a theme following nebular documentation, add a custom property: value

**Related code:**

```
$nb-themes: nb-register-theme((
color-primary-500: #f5548e,
text-primary-hover-color: #f5548e,
), default, default);
```

### Other information:

https://i.ibb.co/cFXMCBX/nebular.png

"@angular/core": "8.2.14",
"@nebular/theme": "4.6.0",

**npm, node, OS, Browser**
```

```

**Angular, Nebular**
```

```