How to enable transparency for background colors other than default?

[totkeks]

Since I don't think this is a bug in WT but a bug in my config, I'm using the Q&A for this.

The issue is that I use a different background color for the active item in the tab completion menu. As you can see the item format in the fourth column and fourth row is currently selected. Unfortunately, it is displayed with an opaque background color. I would have expected that it also uses the opacity of the terminal similarly to the default background color.

Is that something I need to configure in my color scheme used with the terminal? So far, I saw that I can only configure RGB colors, but not RGBA colors.