Review all settings using units

[chris-pearce]

Defining units on settings makes them really rigid, e.g.:

$shell-g-border-radius: 3px;

https://github.com/campaignmonitor/shell/blob/master/src/_settings.scss#L208

If we wanted a larger radius we can't do:

$shell-g-border-radius-large: $shell-g-border-radius + 1;

Let's review all settings with units.

[chris-pearce]

It seems Sass allows this now.