Improve and clean up CSS

[matt-dray]

General: more consistent use of the blog colour #1e8016.

• [x] #1e8016 links
• [x] Better quote boxes
• [x] Better hint boxes
• [x] Better code box
• [x] Better code highlight
• [x] Better table design

[matt-dray]

<th> is white; why does transparent revert to black? Might look weird if someone has dark mode on or something.