Expose color variables to installed cardboard.css.scss

Maybe this is just me, but passing over a few hints for styling will make quick color changes a lot simpler for devs building cardboard apps.  It's a lot easier to set $brand_color when you know it exists, compared to identifying & coloring each relevant class (like I did on my first 2 cardboard apps).
I don't feel like these color vars are necessary to include.