Missing visual indication of custom accent colour

[BrodieRobertson]

Once a custom accent colour has been set, there is no longer visual of which box you're supposed to press to actually set a custom accent colour, you just have to remember that you need to press the final box in the list

Before setting:

After setting:

[git-f0x]

It's present in the designs, but isn't implemented yet. Edit: This is a duplicate of #103.

[WatchMkr]

tracking on https://github.com/pop-os/cosmic-settings/issues/103