Custom style of input fields

I want to add this style to the CSS of the Gwolle guestbook. On mobile phones are the input fields very small. I hade solved the problem by making this style:

@media (min-width: 821px) {
	#gwolle_gb div.label {
		width: 80%;
	}
	#gwolle_gb div.input {
		width: 40%;
	}
}

Where can I put this for effect? In the developer mode in Chrome can I change wp-content/plugins/gwolle-gb/frontend/css/gwolle-gb-frontend.css and it works. But when I change it in the source file it don’t work.

I want to change this code:

#gwolle_gb div.input {
	width: 40%;
	border: 0px;
}

to:

@media (min-width: 821px) {
	#gwolle_gb div.label {
		width: 80%;
	}
	#gwolle_gb div.input {
		width: 40%;
	}
}
#gwolle_gb div.input {
	border: 0px;
}