changing the width of the central part of the post

Hey there,

You could use some CSS like this:

#homepage {
    width: 72%;
}	

#sidebar {
    width: 22%;
}

There is a chance that the theme has a CSS field in the options, if not then you can make a child theme:

http://codex.wordpress.org/Child_Themes
http://premium.wpmudev.org/blog/create-wordpress-child-theme/

Or you can instead use a plugin if you like:

http://wordpress.org/plugins/custom-css-manager-plugin/screenshots/

http://wordpress.org/plugins/my-custom-css/screenshots/

http://wordpress.org/plugins/pc-custom-css/

http://wordpress.org/plugins/imporved-simpler-css/screenshots/

Have a fantastic day! 🙂

Awesome, glad I could help 🙂