ui/theme: configuration option for default theme color

[Forfold]

Right now, the default theme color is always the Material blue. This can be changed from the profile popover user by user, but as a request it would be nice to have a default color set from the Admin config.

Use cases:

• Different environments can be defaults to certain colors. STAGE could be red, PROD could be brand colors, etc
• Companies have different brand colors from each other