Wrong font color in header on the Material Theme

### Issue type

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

* [x] bug report
* [ ] feature request
* [ ] question about the decisions made in the repository

### Issue description

**Current behavior:**
Font color on header texts and icons is gray

**Expected behavior:**
Text color and icons on header should be white as defined in `_material_overrides.scss`

**Steps to reproduce:**
Clone the branch `feat/material-theme` and run

**Related code:**
Text color should be overwritten by `_material_overrides.scss` in

```typescript
nb-layout-header {
nb-actions, .logo-container {
nb-icon, .user-name {
color: nb-theme(color-basic-100) !important;
}
}
}
```

but this override does not happen and the color of the icons and text is `actions-icon-color` that is in the end `color-basic-100`

### Other information:

npm 6.14.18
node 14.21.3
Chrome 112 or Firefox 111
Ubuntu 18.04

**Angular, Nebular**

Angular 13.3.9
Nebular 9.1.0-rc.8