Setting z-index to -1

I would like to set z-index to -1 (default is 10000), but if I put the following code to my custom css plugin nothing happens. I’ve tried different solutions but nothing works, in the end it always gets overwritten by default value 10000.

.1223-wpf-id.dc-floater.left.active {
z-index: -1 important!;
}

help please 🙂

https://wordpress.org/plugins/wp-float/