invalid input

Hello,
In case of invalid input ( In a form ) ( or no input in a required field ) i
change the border of the text box :

form.invalid input.wpcf7-not-valid{
border:1px solid red;
background:#f0e3e3;
}

The problem is that in case of drop down menu the color does not change ( if no value was selected ).

How can i fix that?
Thanks

https://ww.wp.xz.cn/plugins/contact-form-7/