[Feature/Improvement] Add more colors for color coding timers

[a-mabe]

Purpose: As a user, I would like to have a variety of colors to choose from to color code my timers.

More details:

The current color picker uses the material defaults which is a limited selection. Additionally, some of the colors are too light against the default font color as pictured in the screenshot below.

Use the Custom Colors feature of the material color picker to set a list of colors that contrast well against the default font color. Aim for no more than 40, at least 25.

Suggested colors:

1. Colors.red,
2. Colors.red[900]
3. Colors.redAccent[700]
4. Colors.deepOrange,
5. Colors.deepOrange[900]
6. Colors.orange,
7. Colors.orange[900]
8. Colors.amber,
9. Colors.yellowAccent[700]
10. Colors.lime[900]
11. Colors.limeAccent[700]
12. Colors.green[900]
13. Colors.greenAccent[700]
14. Colors.teal,
15. Colors.tealAccent[700]
16. Colors.cyan,
17. Colors.cyanAccent[700]
18. Colors.lightBlue,
19. Colors.blue,
20. Colors.blue[900]
21. Colors.indigo,
22. Colors.indigoAccent[700]
23. Colors.deepPurple,
24. Colors.deepPurpleAccent,
25. Colors.purple,
26. Colors.purple[700]
27. Colors.purpleAccent[700]
28. Colors.pink,
29. Colors.pink[900]
30. Colors.brown,
31. Colors.grey,
32. Colors.blueGrey,
33. Colors.black

Acceptance Criteria:

• There are at least 25 colors to choose from and no more than 40.
• All selectable colors contrast well against the default font color.

[MartinSchott]

It would also be nice to be able to add pictures as backgrounds (I imagine that typical wallpaper pictures could work well and would give the different intervals more individual character)