Support dark mode

[GaryHughes]

.NET 9 adds experimental support for color modes, we should support this. There is a lot of color customisation in the UI which will need to be improved to provide light/dark mode alternatives.

[GaryHughes]

I did an initial pass at supporting this and it is not ready for use. Removing all the custom colours still leaves many controls with wrong colours eg. tab control tabs. There are many open issues for this feature, review in subsequent releases. https://github.com/dotnet/winforms/issues?q=is%3Aissue%20state%3Aopen%20SystemColorMode.Dark&page=1