No rounded workspace preview when thumbnails background is enabled

[denielton]

If the option "restore desktops thumbnails background" is disabled, the workspaces preview is perfectly rounded:

But if I turn it on, even with the line

.workspace-thumbnails .workspace-thumbnail {
  border-radius: 8px;
}

under the file /home/user/.local/share/themes/Adwaita-dark/gnome-shell/gnome-shell.css, the workspaces preview become square:

is there anyway to restore thumbnail background while keeping the rounded edges?

[eobet]

I see the same. It looks weird since the background on the large overview below is very rounded.