Update theme and create a smaller version of the logo

[ramonsaraiva]

Used imagemagick to create a smaller version of the logo: convert -resize 20% -crop +10+14 -crop -10-14 src/logo.png src/logo-small.png, the crop args are to remove unnecessary "border" from the image so it fits better in the header, otherwise it was getting too small.

Not sure if the plan is to simply overwrite logo.png but I thought about leaving it there so we can use it as a source to generate other sizes in the future, if needed.

Header preview:

Docs preview:

Browsed through all pages and it seems to be working well!

Closes #17

[dantownsend]

@ramonsaraiva Perfect, thanks! The logo looks good to me.