DP: Visual Update (paint-only)

[relativityboy]

As a visitor to various Steem sites, I would like to see the developer portal have the same look as sites on steem.com so that I can have more confidence in Steem based on main sites unified, professional appearance.

TBD: It's possible this story is more an epic.

AC

• [ ] Should update the steem logo to the gradient ones on steem.com
• [ ] search bar could be more discoverable. Bright colored search icon (see @pkattera)
• [ ] top right illustration is out of date - source and place new one (see @pkattera)
• [ ] would be more elegant if pages had a fixed width (gutters either side for wide/large)
• [ ] fonts - use roboto. Update others as needed
• [ ] clean appearance of tutorial pages where possible

[relativityboy]

Codewalks may need to eliminate gutters/content margins on small screens (or have a toggle for them to be off)