add documentation for new global theming variables

[brandyscarney]

URL
https://ionicframework.com/docs/theming/themes#application-colors

There have been newly added application colors for theming items on hover/focus: ionic-team/ionic-framework#18606

Or, wait until this issue is done to add them all: ionic-team/ionic-framework#18279

[brandyscarney]

Include ion-backdrop-opacity: ionic-team/ionic-framework#19533

[brandyscarney]

This has been done.