Hello,
In theme package, you can find header-style1.html file.
You can include the content of the file to the other html, instead of original header.
And we used ajax popup to show image gallery popup.
If you upload the html files to your server and visit them with url, not just clicking html files directly, you will see popup.
Best regards,
greeting. i found an html file and figured it out. thanks a lot for the show image gallery popup.