Add custom color option to broadcast messages

Follow-up from a discussion:

I looked through the MR and !83251 (comment 935798883) in particular. I was thinking the original change would've been to remove some of the color options with low contrast, but not all with theming replacing them.

A color option should be added back to the broadcast message create\edit form. An automatic color contrast for the text color should be established. A theme option should be removed and the colors from this option should be used in the color option now.