after_header.html テーマテンプレートについてお手伝いが必要です

For future reference, it looks like the issue was here:

<div> {{about-site.description}} </p> </div>
                                   ^

You had a stray closing tag, which wouldn’t immediately break your site if it was plain HTML, but will cause an error and fail to render in handlebars.

In future, you can check your browser console (Ctrl + Shift + J in Chrome) to check for any error output.