Unable to control checkmark's behaviour with CSS. Ex: Checkbox, Toggle (when checked), etc.

### Issue type

**I'm submitting a **

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
I am not able to change the checkmark's look/behaviour (ex: make it flat, reduce it boldness, etc.) apart from the `font-size`. Since it is SVG, I am restricted to do anything w.r.t. styling with SCSS.

**Expected behavior:**
Should be able to control the `font-weight` and related properties as we can do for Radio Buttons (since it is not an SVG but, normal HTML elements).

**Steps to reproduce:**
Try making the Nebular checkbox's checkmark or toggle's checkmark as per the attached images.

**Related code:**
Not needed. Please refer to the below 2 images. Base font-size is 1rem.