Update dialog to modern CSS

[jon49]

Since Pico was originally created CSS has advanced quite a bit. It would be nice if Pico added some of those new features. One of those features is having a blurry background just for the dialog element. Also, the ability to click out of the dialog element and it auto closes the dialog. These features fit Pico's vision in doing as much with CSS as reasonably possible without JS.

[tonystar]

added to Roadmap of Blades CSS — a fully compatible and actively maintained successor to Pico CSS:

anyblades/blades#136