[Oreo] update dark mode colors

Description

We modify the dark mode colors in order to improve it.

Issue(s)

https://gitlab.e.foundation/e/backlog/-/issues/3869

Test environment

1. Run a build with
   • BRANCH_NAME: v1-oreo
   • FEATURE_BRANCH: 3869-o-update-dark-mode-colors
2. Install it on the device (update or fresh install)
3. Switch into dark mode (Settings > Display > Dark mode)

Test procedures

Confirm that the following application looks good in dark mode, according to the guidelines shared at https://www.figma.com/file/aPb7JkftqdgnxJgYfTKEmL/Dark-Theme-Material-Design---%2Fe%2F-Fork?node-id=108%3A5830

Technical details

The modification should improve the dark mode by

• a better UI (better colors)
• a darker UI
• a better readability (more contrast)

Screenshots

Check list

• Self review
• Test procedure explained
• Tested on fresh install
• Tested applied as an update
• License
• Internal documentation
• User documentation