Hi. I'm trying to upgrade my Joomla 3.10 site to Joomla 4.4, with the J51_stockholm template for Joomla 4. I was getting a 500 error ("500 - behavior::caption not found"

FYI I'm testing this on XAMPP under Windows 11 and also on a subdirectory of my website hosted at Siteground. I get the same results on both.
I also posted the issue on the Joomla forum, and got the following response from a Joomla Master:
Post by toivo ยป Tue Jan 09, 2024 4:22 pm
2 Joomla\CMS\HTML\HTMLHelper::_() JROOT\templates\j51_stockholm\html\com_content\featured\default.php:14
The template has an obsolete override that in its html folder. Rename the folder templates\j51_stockholm\html\com_content.
That workaround should fix this issue, other may pop up. It would be better to use a template that is compatible with Joomla 4, like Cassiopeia.[*]
When I renamed the folder to templates\j51_stockholm\html\com_content2, the Home page appeared, but it has lost format details. I assume there's a better way to fix this.
Here's the beginning of the debug list:
behavior::caption not found.
C:\xampp\htdocs\J4\libraries\src\HTML\HTMLHelper.php:141
Call stack
# Function Location
1 () JROOT\libraries\src\HTML\HTMLHelper.php:141
2 Joomla\CMS\HTML\HTMLHelper::_() JROOT\templates\j51_stockholm\html\com_content\featured\default.php:14
3 include() JROOT\libraries\src\MVC\View\HtmlView.php:415