if you do not have a current copy of phpbb on your computer, you should.
download it from here ( click the downloads button at the top of every page here )
then, unzip it on your computer. then using ftp only, upload the prosilver style to the styles folder on your server.
your current overall_header.html file is missing most of the code that needs to be there.
if you are trying to create a custom header there are plenty of instructions here to help you.
for one thing, you should not remove anything from the <head> section of the page.
anything you need to edit would be in the <body> section, not the <head> section.
I am going to be out of town and off line for a week . see ya when I get back.
Premium phpBB 3.2 Styles
If a tree falls in the forest and nobody is there, does it make a sound?