.NET 7: Document dark mode for developer exception page

[Rick-Anderson]

See dotnet/aspnetcore#39676

@poke how do you enable dark mode?

[poke]

@Rick-Anderson Dark mode is enabled automatically if your browser detects that you prefer dark mode for websites. By default, this happens when your system preference is set to dark mode but browsers will usually also allow you to override this in their settings.

If you want to test this, you can also switch to dark mode with the browser developer tools, e.g. in Firefox:

Or in Chrome:

[Rick-Anderson]

FF

Chrome

[Rick-Anderson]

Added to What's new
Thanks @poke