Establish colors & typography for dukespace theme. Closes DUK-25.
- set color name variables, including palette from Duke Brand Guide
- establish main content section in white box against whisper-gray page body
- make search term hit-highlights more highlighted
- improve styles for admin UI sidebar
- increase displayed thumbnail img size
- add tint/shade functions for better color control
- add mixins for semibold font and accessible underlined links
- slightly widen the BS4 grid breakpoints and max-widths (match DDR)
- minor tweaks to borders, margins for improved aesthetics and accessibility