that is basically what you need to do.
wordpress is not coded the same way as phpbb is. in wordpress all the html and php are in the same files.
that makes it harder to work with . in phpbb the html, css, and php are all in separate files.
anyway, view the source of your main wp page. get only the header part of the code.
copy that and paste it into a text file.
by the way, only use a text editor for working with phpbb, any other type of editor will corrupt your files.
now, download your style's overall_header.html file found here:
open that file in your text editor.
now paste the header code right after the opening <body> tag of your overall_header.html file.
you will have to tweak the image paths, css and other things to make it all work properly but that is the basics of how you do it.
I would suggest that you install a webserver on your computer:Installing and Setting Up Your Own Web Server
then, install wordpress and phpbb on your new webserver.
install the same themes/styles that you are currently using.
then, you can edit and tweak to your heart's content with no effect on your live board.
once you have the style working as you wanted. you simply upload the style files to your live server to replace the ones there and you are done.
I am available for custom work on a donation basis. Please shoot me a Pm with your needs.