gnome-shell/subprojects/extensions-app/data/css/style.css

row.extension>box {
 margin: 12px;
}

row.extension>box,
row.extension box.header {
  border-spacing: 12px;
}

row.extension box.actions,
row.extension box.actions>box {
  border-spacing: 6px;
}

row.extension box.information,
row.extension box.status {
  border-spacing: 3px;
}