I suppose that depends on the file that is uploaded. The far-too-obvious-likely-not-what-you-meant answer is reference the style sheet in the html of the file that gets uploaded.
<link rel="stylesheet" type="text/css" href="http://www.mysite.co.uk/styles.css" />
If you mean to insert your stylesheet into every html document uploaded, you'd most likely use something like fgets to read each line and insert that line after you find a line ending with "<head>" perhaps.
Or perhaps you load all the file contents still and use a global expression with preg_replace().