New navigation themes in GitLab 11.X

Description

In GitLab 11.0, we introduce five additional great navigation themes, including light versions of existing themes.

We could add additional light themes in an upcoming release.

Proposal

Staying on top of providing a more personalized visual experience, there is the potential to add a few(!) additional unique color themes that do not have to relate to what we had previously.
Attached table shows Pink and Cyan prototype screens based on Material color palettes.

Pink	Cyan

Open: Not necessary, but we could add dark versions of both themes.

cc @dzaporozhets to discuss tomorrow.