[Design Flaw] Certain variables cannot be overridden.

If you want to override the media query variables:
```
$small-screen: emCalc(768px) !default;
$medium-screen: emCalc(1280px) !default;
$large-screen: emCalc(1440px) !default;
```

you need to define them before loading `_foundation-global.scss`. The problem here is that you will need the `emCalc()` function if you are to do this correctly.