Allow customizing server folder background color for better contrast

We can customize a server folder color to identify it quickly, but not a server folder background color. While not very useful when the folder is folded, when it is unfolded, I noticed that the servers belonging to that folder are placed on a "folder background" whose color is very close to the rest of the left bar's color in Dark Mode. As a result, it is difficult to tell where the group ends and if a server below belongs or not to the folder.

Below, the server is not part of folder

Below, it is part of folder

(EDIT: for some reason, my uploaded pictures don't show properly, you'll have to try it yourself, by creating a server folder of 2-3 servers, then drag the last server of that expanded folder outside and back inside the folder and compare visuals)

Note that very low difference of contrast.

I suggest the following:
a. make sure that default server folder background contrasts with left bar background with all themes
b. allow user to customize server folder background

One of the two would fix the issue, and I fix a. is more important to do first as it avoids having to set all background colors manually.
An alternative could be to automatically set the background color to some color related to the folder color; but it would have to contrast with both the folder color itself, and the left bar background color.