Full Width child theme

Hey guys! I have a little issue here.
I am using a Chid Theme on my website, the parent theme is Opportune and I am struggling to make the site full width.
The theme offers an option for full width, that box is checked, but still there are left and right white spaces left.
I mention I am new to WordPress, so I don’t know any CSS.
Someone adviced my to copy-paste this CSS code in style.css of my child theme, but nothing happened.

@media (min-width: 1200px)
.container {
width: 100% !important;
}
@media (min-width: 992px)
.container {
width: 100% !important;
}
@media (min-width: 768px)
.container {
width: 100% !important;
}

Many thanks!