I have successfully upgraded my Joomla! 1.5.26 website to 2.5.24 (Core files only; will upgrade modules once everything is working) and it looks fine with a preinstalled Joomla! template. However, even though I feel I have followed all the steps to upgrade my custom template, it still shows as a blank page on the front end. This is not an issue involving an HTML file overriding a PHP file. It is definitely the custom template.
Here is the website the way it is supposed to look (version 1.5.26):
Here it is after the upgrade (version 2.5.24):
Please review the attached files. Thank you.