Smart-table actions button <nb-icon>

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
My project is importing https://github.com/akveo/nebular/blob/master/src/framework/theme/styles/global/tables/_smart-table.theme.scss
and SmartTable actions font is too big, is that right?

In demo project they are using icons, so i get why font size is so big
```
edit: {
editButtonContent: '',
saveButtonContent: '',
cancelButtonContent: '',
},
```
This dosen't work, and changing it to
```

```
the icons dont show either, don't know if in doing something wrong;
On StackOverflow:
[https://stackoverflow.com/questions/57036330/how-to-display-custom-icons-on-ng2-smart-table](https://stackoverflow.com/questions/57036330/how-to-display-custom-icons-on-ng2-smart-table
)
work just fine outside smartTable
How can i make show?

**Expected behavior:**
Normal Styles

**Steps to reproduce:**
Add smartTable to a nebular project

```
th.ng2-smart-actions-title-add {
a {
display: flex;
align-items: center;
justify-content: center;
**font-size: 1.75rem !important;**
}
}
```