This introduces basic styling for modal boxes to support dark mode. It

uses the Ink container styling for the main box, and Ink header styling

for the title box.
As modalbox doesn't actually control other elements of the page, this

doesn't do anything to style buttons or other elements. That's up to the

caller.
There are long-term plans to move modal dialog functionality directly

into Ink, at which point modalboxes in Djblets will be deprecated.

Tested with various dialogs in Review Board (which are undergoing

updates).