Logging in to guardian theme

i downloaded a new theme a few weeks ago, and I’m just now getting around to building the site. i tried logging in to my site.com/wp-admin and this is what i get.

Warning: Cannot modify header information – headers already sent by (output started at /home/content/05/10240205/html/somdwebdesigns/wp-content/themes/guardian/customizer.php:837) in /home/content/05/10240205/html/somdwebdesigns/wp-includes/pluggable.php on line 1228

any other ideas on how to log in on this free theme from weblizar.com