fix(brand): Resolve colors in typography before sending to Sass vars

[gadenbuie]

Small fix to resolve colors in brand.typography.*.{color,background-color} before sending them to Sass.

Prior to this change the following yaml

color:
  palette:
    bright_pink: "#FF3D7F"
    orange: "#FF6F20"
    gray_light: "#E0E0E0"
  primary: orange
  danger: bright_pink

typography:
  headings:
    color: primary
  monospace-inline:
    color: bright_pink
  monospace-block:
    background-color: gray_light

would set

$headings-color: primary !default;
$code-color: bright_pink !default;
$pre-color: bright_pink !default;
$pre-bg: gray_light !default;

This PR calls brand.getColor() on the value of color or background-color properties before passing to Sass.

[cscheid]

This fix exactly was on my plate for today. I'd have implemented it exactly like that. Thank you!