Add `uw-colors.scss`, continue to tweak colors

This PR continues to tweak Scout to use UW brand colors. A new uw-colors.scss file has been introduced to act as a central location for UW color variables. Some screenshots:

On hover, the "Reset" button switches from "UW Red Dark" to "UW Red":

More like this to come.

Please review: @NUWAN.KUMARASIRI @TREFILEK @bjsousa