Use red/strawberry accent color

[fabiozaramella]

Proposal

We should use red as the accent color to match the app's icon color.

Prior Art

Calendar uses green, Music uses orange, AppCenter will probably adopt purple so it makes sense to use strawberry here. Also because we (fortunately) droped the idea to use colored titlebars long time ago, so it would be nice to use accent color in branded apps where possible.