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Ok, first of all don't panic
This will get you nowhere.
Right. Make sure above all that your theme.html file is properly structured, that is:
<html>
<head>
<title>title>
head>
<body>
body>
html>
That is the basic structure minus of course all the divs the meta tags and so forth.
After making sure that this is the correct structure, link properly to your stylesheet. There are 2 ways of doing so. Either:
<link rel="stylesheet" type="text/css" href="path/to/style.css" />
or...
<style type="text/css" media="all">@import (/path/to/stylesheet.css);style>
The "media" property can have different values but you can safely set it to "all" for now, that is, if you decide to link to your stylesheet that way.
Also, don't forget! Any link for styles, meta tags and so forth go between the "head" tags.
Once you have that straightened out, copy the style rules for the phpBB theme you are using to your style.css file and link to them from there.
First things first though. Make sure that your markup is correct!