Cant style

Dear, I love to use your plugin but recently I having an issue with styling contact form. I made child theme of my template and I added code below in stile.css of my child theme. I dont know why nothing happened and it was still in same style. Then I put this code to style.css of my main theme and It works perfect. But I know that this is not proper becouse when theme will be updated I will loose all my styling becouse it will be owerwritten.
Im having question where to put this code that it will stay for ever.

Second question is about rounded edges od form. Is it possible to put css code that make for example 2px radius edges?

My website is: http://fizio-terapija.si/izdelek/magnetni-trak-za-vrat/

Best regard

Code is:
.wpcf7 input[type=”text”],
.wpcf7 input[type=”email”],
.wpcf7 textarea
{
background-color: #fff;
color: #000;
width: 90%;
}
.izberi select {
max-width: 230px;
}
.wpcf7
{
background-color: #6dbcdb;
border: 2px solid #575a59;
}

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