* Make sure the path is correct which is $XDG_CONFIG_HOME/wofi/style.css (XDG_CONFIG_HOME defaults to $HOME/.config if it is unset).
* Try to specify it manually with -s option from the terminal and see if it works or gives any errors.
Other than that, I have no idea. I also installed it same as you and it works fine on my machine.
-- Best Regards, Oğuz