Rainbow pen animation should be changed as it becomes almost invisible at times with colour themes.

[TheTeaCat]

[onfe]

If we force icons to always be black for contrast we could rainbow cycle the button background?

[TheTeaCat]

Would rather not force colours. 2 ideas:

1. Add a border alike the announcement pip
2. Change the animation to be a shifting gradient instead of block colour (pretty sure this was the solution I had in mind when I created this issue)