Set all font-sizes relative to $font-size-base
Created by: tillmon
If all font-sizes are relative to $font-size-base, it would be simpler to change overall font-size:
$font-size-base: 1rem !default;
$font-size-lg: ($font-size-base * 1.25) !default;
$font-size-sm: ($font-size-base * 0.875) !default;