Hi the problem is HTML markup, the code you posted, which will likely be deleted with a message to use pastebin!
<div id="footer" role="contentinfo">
<div id="colophon">
These are two open tags that are not closed and will leave both the #footer and #colopone elements open.
That is not good so we have no idea where the issue might be, as the default theme was changed and not a child theme it becomes very hard if not impossible to help!
Download the twenty eleven theme from the themes directory above and compare the code in header.php and footer.php and look to find open tags that are not closed.
Polite Suggestion:
It might even be easy to just re-install the twenty eleven theme from the admin area, this will delete the changes and revert back the theme, then create or download a child theme, install and activate the child theme, then post back here what you want to do, which looks like hide the footer credits, this can be done really easy in the child theme style.css
#colophon {
display: none;
}
Change to Dark Theme Option from Appearance > Theme Options
HTH
David