Refine "Add new block" indicator style

<img width="445" alt="image" src="https://user-images.githubusercontent.com/1200432/59795972-1cf9c900-92a2-11e9-97d9-8fc662deec10.png">

Requested Changes:

* Text color: `$gray`
* Font-family: system `sans-serif`
* Font-size: `14px`
* Text-transform: `uppercase`