[Plugin: Contact Form 7] making fields rounded?

That’s a CSS issue – not a plugin one. You need to style your input boxes via your theme’s stylesheet.

How can I do that?

input {
-moz-border-radius: 5px;  /*or whatever radius*/
-webkit-border-radius: 5px;  /*or whatever radius*/
}

https://www.css3.info/preview/rounded-border/