Allow to customize brand color for project as previously

[bjarnef]

Previously in Umbraco 7 and 8 it was possible to customize brand/theme color for project via environment config.
Unfortunately this feature was removed with support for Umbraco 9+ / .NET Core on Umbraco Cloud.

I suggested it 3 years ago, but it seems there isn't any news regarding this:
#579

Sadly the random color often doesn't match the brand and worse on a recent project the contrast between text and background color is really bad umbraco/Umbraco.UI#1089

Could we either adjust this vis environment config or a setting on the project in the portal?

I also noticed a bit inconsistency. Some projects we have are two words, but use single letter in avatar in overview, while others use two letter, but we also have a project with in one word, but still two letter avatar.

Another project name as single word use single letter in avatar in overview and project page, while another one using single word has two letter avatar.
Perhaps it depends on when the project was created. I am not sure if the letter is stored somewhere and independent on future UI library updates like change here: umbraco/Umbraco.UI#1067

For a single word I think it makes sense to use single letter.
For multi words, I think it is common to use first character in first and last word in avatar only support two letter (at least in this example https://react.fluentui.dev/?path=/docs/components-avatar--docs and more than three words just use single letter in avatar), but with the change in UI library it may be possible to adjust in the portal in future.