Built-in Restricted Access page?

The old plugin used unsupported APIs, sometimes caused issues, and there isn't a clear upgrade path for it. I'd recommend we use a built-in approach. That said, what level of customization would a customer want? Would being able to supply colors and a logo in addition to labeling be enough? Worth noting that, like the old maintenance page, this new built-in preview-mode middleware necessarily runs outside of the typical theming/widget/styling/API context.

I think at a minimum they'd like to set the background colour and add their logo, so it at least carries their brand. The text side of it already seems to be covered. A plain grey page with a big exclamation isn't very inviting and can be quite jarring. Here is an example of one customer's site although they don't have the logo.

https://macmillan.test.4-roads.com/