Is your feature request related to a problem? Please describe.
In modern design transparency background colour is used frequently (e.g. sub-blocks of text in a group block). On all blocks that provide a background colour the GUI control supports colour adjustment but not transparency adjustment. This forces the user to add custom CSS for a trivial action which is counter to the intention of GB to be Wix-like and useable by ordinary consumers.

Describe the solution you'd like
On all blocks that provide a background colour the GUI control should include a transparency slider (like the opacity slider for the background image of the cover block).

Describe alternatives you've considered
Unless there is a reason I am not aware of the technical implementation would be changing the background colour setter to use CSS RBGA vs. RGB as at present. The transparency slider control should be able to reuse the opacity slider currently in use by the Cover block.