Theme: Twentyeleven – Maximun Width

It is doable.

You can copy what ever is in style.css and paste it in your Child Theme stylesheet. Then change what ever you want in your Child Theme, e.g from 900px to 1200px.

Make sure to delete styles you’re not changing.

You don’t need to copy your stylesheet from the theme into the Child Theme, you can just import the styles over, as seen in this example by Codex.

Are you saying that everything that is NOT changed in the child-them stylesheet should be deleted?

I meant if you’re going to copy line 71 from style.css from the main theme

#page {
margin: 0em auto;
max-width: 900px;
}

into the Child Theme

#page {
margin: 0em auto;
max-width: 900px;
}

you can do so but if you’re only changing the width, you can remove the margin style as it’s only going to clutter up your website

#page {
max-width: 1200px;
}