tutorials: Fix a few minor style glitches
tutorials: Fix a few minor style glitches

- Removing uppercase of table headers, which is unnecessary
- Reducing `line-height` to standard values
- Replacing custom light grey with WikimediaUI Base90
- Reducing footer height and header margins
- Reducing pre margins for better readability and layout structure

Change-Id: I418c16a8de9b6a68b8304ac3c5db8f0bb569e6e6

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui