Dialogs should be of slightly different colour to the layer below.

Basically what I'm trying to say it that if I have an application that is made of let's say panels, and then I add a Dialog via siv.add_layer, the dialog is simply of the same colour as the layer below and it simply looks bad.
The problem here is that unlike with GUI frameworks where we have so called active and inactive (but not disabled) window state, here in cursive this is simply not the case.
There are workarounds of course, but in my opinion this should be natively supported by library.

Thank you

Start by tracing how siv.add_layer renders dialogs and how colors are selected for the active and underlying layers. Define the supported behavior and configuration needed so a dialog appears in a distinct color from the layer below, then verify it in a representative terminal UI case.

Written by the indexing model from the issue text.