Improve visuals for dialogs

[chrisglein]

WinUI dialog:

Current dialog:

Can't get that bottom solid color because the corners need to be rounded on bottomLeft and bottomRight but not the top edge. And Windows doesn't support that :/

Having the default button selected would be good. But the RN Button has a color property that sets the background but doesn't update the foreground to match (and keep contrast ratios clean).

The icons are very nonstandard and I should probably drop them.