Fileupload widget Uploaded file style

I have the following file upload widget and i want to edit the background of the uploaded file part.

I have tried using the custom css  and setting list-item in the customClass property

.list-item::part(file-item) {
 background:#383838;
}

 following the ptc documentation I tried also this

@supports (ptcs-style-unit: "PTCS-FILEUPLOAD") {
 [part=file-item] {
	background-color: #383838; 
 }
}

but all my test failed to edit the style of that part