Removing Empty Sidebar

You don’t want to modify the theme files directly. If the theme gets updated because of feature enhancements, bug fixes, or security patches, or if the theme has to be updated because of a change to the WordPress core, then your changes will be lost. Instead, either create a child theme or use a CSS plugin like Jetpack or Custom CSS Manager. Since what you need to accomplish can be done using just CSS, I would suggest using a CSS plugin rather than creating a child theme. Unless your theme has some sort of custom CSS option, however; in that case, you can use that instead.

Whichever route you pick, you want to add these CSS rules:

#secondary {
    display: none;
}
#primary {
    width: 100%;
    padding: 0 100px;
}

The first rule hides the sidebar and the second rule expands the main content area to fill in the space left by the sidebar. You can adjust the value for padding, which is the amount of empty space between the text and the left & right sides of the content area.

@puhelin09, you can’t necessarily take the CSS for one theme and use it on a different theme and expect the same results. Theme designers often use different class names or ID for their elements. Second, you shouldn’t directly edit any theme files like page.php, because when/if the theme gets updated, those changes will be lost. The suggested method for making those types of changes is to create a child theme and make your changes to a copy of whatever file you want to modify.

For Twenty Ten, the sidebar is the element which has an ID of primary, so to hide the sidebar and stretch the main content out to the full width, use this CSS instead:

#primary {
    display: none;
}
#content {
    margin: 0;
}

For other questions about the Twenty Ten theme, please post them on the Twenty Ten support forum.