transparency in highlight

[vespertinee]

hello! i'm trying to achieve an effect i saw in one of the official screenshots for this program

https://github.com/complexlogic/flex-launcher/raw/master/docs/assets/screenshots/screenshot1.png

in which the highlight (currently over the ''media'' option) shows an effect of transparency so you can see the outline of the background behind it (in that case, the beach or the coast) i messed around with every variable in the .ini under the [Highlight], but i couldn't achieve it.

can anyone explain how do you do it? thanks!