$base_font_size is overriden in _media-queries.scss

Similarly to https://github.com/johnotander/pixyll/issues/196 - there's a variable in `_variables.scss` for `$base_font_size` (14px by default), but this is overriden in `_media-queries.scss`:

``` sass
@media screen and (min-width: $viewport-large) {
html {
font-size: 20px;
}
}
```

I'm guessing this should be expressed in terms of the `$base_font_size` - e.g 1.5rem for simplicity (21px) or 1.4286rem for exactness (20.0004px)