Fixed bugs and added functions

【Contents of the fix】
■Fixed a bug where the link buttons for “Next Article, Previous Article” did not display properly when displayed in tablet size.
■Added custom script function.

The following are the files that have been modified for this update.
【Modified file】
admin/theme-options.php
functions/head.php
functions/next_prev.php
languages/ja.mo
languages/ja.po
responsive.css
style.css