Revert step title position to horizontal
Adjust gradient of the "blue" boxes

From Dave:

 "Maybe it's the desaturated/dark color being at the top or
 maybe it's the hue clashing with other elements of the poster.
 [...]
 Please consider something like #3d71ae (top) #4686c6 (bottom)"
Remove middle boxes so that CMake and Python logo are more preeminent.

Also adjust size of github url and position of the github social icon.