Use red/strawberry accent color

## 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.

The issue names no files, tests, or entry points. Start by locating how the app currently defines its UI colors and compare the Calendar and Music conventions mentioned in the proposal; done means the app consistently uses a red or strawberry accent that matches its icon.

Written by the indexing model from the issue text.