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 waffled on this one, but since $main_sidebar_background and topbar_color reference the color vars, I threw them in. I'm not going to fight anyone that takes them back out, though.