Add custom colors to profile manager

[TWB0109]

The new profile manager is really good. Kudos! 

I like that when colors are selected, I can switch from dark to light mode and it will do the switch, but I would love it if I could select a custom color. See side by side with chromium:

In Chromium I have selected a color that better matches the theme of my desktop, I'd love to be able to do this in Firefox.

[Jon]

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