Want To Clean up Error and make responsive

Hello There,

I would like to clear up what seems to be the only error reported by w3c

http://validator.w3.org/check?uri=http%3A%2F%2Fwww.hellswinter.com%2F

I would also like to have the template made to be responsive for mobile devices. I am lacking on nibbler.

http://nibbler.silktide.com/reports/www.hellswinter.com

Can you please advise, help, inform me of cost to have template brought up to the next step. Also, theme checker is advising:

RECOMMENDED: Text domain problems in zombie-admin-functions.php. You have not included a text domain!

Line 589: wp_die( __(‘Sorry, but you do not have sufficient permissions to access this page.’

WARNING: The flexible-width and fixed-width tags changed to fluid-layout and fixed-layout tags in WordPress 3.8. Additionally, the responsive-layout tag was added. Please change to using one of the new tags.
RECOMMENDED: Screenshot size should be 880×660, to account for HiDPI displays. Any 4:3 image size is acceptable, but 880×660 is preferred.
INFO: Possible hard-coded links were found in the file zombie-admin-functions.php.

Line 595: <p style=’display:inline;float:left;’>Customize your Zombie Apocalypse Theme to suit your needs. Visit the theme’s home page for more inf

Thanks so much for a great template. I would appreciate the help.

Kind regards,
Ryan Kuhn