Multiple bug fixes

【Contents of the fix】
■Supports the “Contact Form 7 Multi-Step Forms” plug-in.
■Fixed a bug where the “Return to Top of Page” button was displayed at the bottom of the page when the loading screen was displayed.
■Fixed a bug that caused the menu to collapse when viewing the site with Safari browser.
■Fixed a bug that prevented the contents of the showroom archive page from being displayed when the loading screen was displayed.
■Adjusting the layout of the official Twitter button

【Modified files】
css/design-plus.css
css/sns-botton.css
functions/footer_script.php
style.css