Enable custom colours for the banner component

[oran-nitro]

Right now, the colours used for a banner (specifed in docs.json) is wholly dictated by the theme.

It would be good to be able to customise the colours of the banner, either via the colors block or directly in banner

"banner": {
    "content": "Welcome to my documentation",
    "dismissable": true,
    "bgColor": "#000000",
    "fgColor": "#ffffff"
}

[oran-nitro]

Definition may need to be richer to allow for different colors dependent on light/dark theme.