You’ll need to apply a different background color in order to set up transparency for a button. With the defaults, the transparency settings don’t have a color to reference for say the border color, etc. Give it a shot and you’ll see what I mean, bit hard to explain.
Yes it’s hard to explain as in “not very transparent” But I got it working.
This issue came about because the Accent Color is set for both text and button backgrounds. So I set the Accent Color to “Orange” for the button background and did a bit of CSS for the Text links (“Blue”). Because I wanted “White” text for the button (it kept defaulting to a Dark color), even more CSS.
So in any future Theme changes, I think it would be useful in the Customizer to have the Accent Color for the text links and additional settings for Buttons with colors, sizes, etc. I find that I usually want to target different colors for each. I’ll put that into a feature request.