mixins should allow argument to set `background-color` instead of `color`

e.g. the `color_backgroundXXX` mixins currently set `color: XXX`, when they should really be setting `background-color: XXX`

pseudo-code for generated mixin markup:

```scss
@mixin color_backgroundContent($style: 'color') { #{$style}: $C_backgroundContent; }
```

I'm not super familiar with the SASS builder for this, but I'd like to try making the fix.