phpVMS 5.5.2 template files end in ".php", not ".tpl". For skins, you can mostly just change the file names from .tpl to .php -although you may have to look in each skin file and change any reference from .tpl to .php also.
Skins made specifically for phpVMS 5.5.2 will not require this modification.
Some older skins made for phpVMS 2.x were also converted for use in phpVMS 5.5.2. Other skins sometimes come with the files for both versions.
You may be best served initially to find a skin made specifically for phpVMS 5.5.2 - such as ones available at 209 studios.
https://phpvms.209studios.com/downloads.php