Some theme properties like `button-medium-icon-size` does not apply to registered 3rd-party icon set

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description
Some theme properties does not apply to icon set other than the eva-icon set.
Although 3-rd party icon set, font-awesome, is registered in app component, properties like `icon-font-size`, and `button-medium-icon-size` where nb-icon is encapsulated inside nb-button does not apply.

**Current behavior:**

Properties like `icon-font-size`, and `button-medium-icon-size` does not apply to font-awesome icon set.

**Expected behavior:**

Should update the icon font-size.

**Steps to reproduce:**

Use nb-icon inside button.

**Related code:**

Registered in app component:

Sample usage.

### Other information:

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

Node: v16.13.0, npm: 8.1.0
OS: Linux (Ubuntu 18)
Browser: Chrome/Firefox/

```

**Angular, Nebular**
```
Angular 13
Nebular 9.0.0

```