Error compiling scss

```js
@import '~bootstrap/scss/bootstrap';
@import '~shards-ui/src/scss/shards';
```

I receive the following error located here

https://github.com/DesignRevision/shards-ui/blob/master/src/scss/_variables.scss#L744

```
$popover-arrow-outer-width:           ($popover-arrow-width + 1px) !default;
                                     ^
      Incompatible units: 'px' and 'rem'.
```