hi, you can't do it that way. the template files are not normal html.
they are a combination of html and php variables and css .
what you need to do is either work on a copy of the style on the live server or install web server software on your computer to run a copy of phpbb with the style installed.
remember, the style version must match the phpbb version to work properly.
you can install xamp or wampserver or usbwebserver etc. you can google for those.
very simple one click installs and then you create a database using the builtin phpmyadmin and copy over the phpbb files and run the phpbb install and done.
Knowledge Base - Installing and Setting Up Your Own Web Server
Knowledge Base - Tools needed to set up and customise phpBB
forgot to mention, the index.htm files you find in phpbb are only blank files so that the contents of a directory can not be views by just accessing it.
they are not usable for any other purpose.