[feat] pfe-modal | add ability to give pfe-modal a dark background

[mwcz]

I'm attempting to make a barebones dark theme for arcade.redhat.com, and pfe-modal is presenting a small obstacle. pfe-modal's background is colored with --pfe-theme--color--surface--lightest and it feels wrong to set that to a dark color. Easy to work around, but the name of the background variable could be more abstract.

[starryeyez024]

This is tangentially related to issue #1090.

Per this comment:

it feels wrong to set [--pfe-theme--color--surface--lightest] to a dark color

probably it should have a local variable like --pfe-modal__content--BackgroundColor.