Fixed broken layout of 404 page and changed hamburger menu specification for smartphones

【Contents of the fix】
■Fixed a broken layout on the 404 page.
■Changed the specification of the hamburger menu behavior when using the back button of the browser on smartphones.
■Added some wording to TCD theme options.

The following files have been modified for this update.
【Fixed files】
admin/inc/basic.php
functions/head.php
languages/ja.mo
languages/ja.po
style.css