Steps to reproduce the issue

Install a new template on Joomla 3.7

Try to open the preview using the URL such as:
http://joomla37/?template=mytemplate

Expected result

The frontend uses the latest installed template named 'mytemplate'

Actual result

The default template is still used

System information (as much as possible)

The code change in site.php included in the function 'getTemplate' caused the regression.

Working code on Joomla 3.6.5:

if (!$templates = $cache->get('templates0' . $tag))
		{
			// Load styles
			$db = JFactory::getDbo();
			$query = $db->getQuery(true)
				->select('id, home, template, s.params')
				->from('#__template_styles as s')
				->where('s.client_id = 0')
				->where('e.enabled = 1')
				->join('LEFT', '#__extensions as e ON e.element=s.template AND e.type=' . $db->quote('template') . ' AND e.client_id=s.client_id');

			$db->setQuery($query);
			$templates = $db->loadObjectList('id');

			foreach ($templates as &$template)
			{
				$registry = new Registry;
				$registry->loadString($template->params);
				$template->params = $registry;

				// Create home element
				if ($template->home == 1 && !isset($templates[0]) || $this->_language_filter && $template->home == $tag)
				{
					$templates[0] = clone $template;
				}
			}

			$cache->store($templates, 'templates0' . $tag);
		}

		if (isset($templates[$id]))
		{
			$template = $templates[$id];
		}
		else
		{
			$template = $templates[0];
		}

Regression code on Joomla 3.7, the 'by reference' variable $template is overwritten in the final $template = $templates[0]; so the latest template style is lost in the $templates array.

if ($cache->contains($cacheId))
		{
			$templates = $cache->get($cacheId);
		}
		else
		{
			// Load styles
			$db = JFactory::getDbo();
			$query = $db->getQuery(true)
				->select('id, home, template, s.params')
				->from('#__template_styles as s')
				->where('s.client_id = 0')
				->where('e.enabled = 1')
				->join('LEFT', '#__extensions as e ON e.element=s.template AND e.type=' . $db->quote('template') . ' AND e.client_id=s.client_id');

			$db->setQuery($query);
			$templates = $db->loadObjectList('id');

			foreach ($templates as &$template)
			{
				// Create home element
				if ($template->home == 1 && !isset($template_home) || $this->_language_filter && $template->home == $tag)
				{
					$template_home = clone $template;
				}

				$template->params = new Registry($template->params);
			}

			// Add home element, after loop to avoid double execution
			if (isset($template_home))
			{
				$template_home->params = new Registry($template_home->params);
				$templates[0] = $template_home;
			}

			$cache->store($templates, $cacheId);
		}

		if (isset($templates[$id]))
		{
			$template = $templates[$id];
		}
		else
		{
			$template = $templates[0];
		}

Additional comments