Add overlay color

[JimmyHoenderdaal]

I've added the overlay color to the color variables so we can use this instead of bg-black/40.

Still figuring out whether we should include the opacity in the variable or add it when we use the variable (bg-overlay vs. bg-overlay/40).

Adding it when we use it could lead to inconsistencies, but having it in the config might mean less control in some cases.

[Roene-JustBetter]

I think it should work like this: bg-overlay/40.