Add subtle borders to widgets (#167773)

* Add border to quick pick to match menus

* Add more borders and border radius to widgets

* More border updates

* Move color defaults to themes

* Address feedback

extensions/theme-defaults/themes/light_vs.json

@@ -29,6 +29,7 @@
 		"list.activeSelectionIconForeground": "#FFF",
 		"list.focusAndSelectionOutline": "#90C2F9",
 		"terminal.inactiveSelectionBackground": "#E5EBF1",
+		"widget.border": "#d4d4d4"
 	},
 	"tokenColors": [
 		{