Updated all the places in the SCSS where we use Darken and Lighten properties.
These properties have been problematic in the past and caused some state issues, e.g. New message(s) hightlight (which was fixed seperately from this issue)
/cc @carljbowman
One issue we ran into is that our `frameless` configuration is in `em` rather than `rem`, making it difficult to have an aboslute grid applied uniformly at all hierarchies. For now, we're using straight `rem` calculations instead of `$cols{1,8}` in the styling until we can refactor `frameless` to use `em`.
