because the content type is essentially forcing a download, having a relative path to your css, such as
<link rel=stylesheet type=text/css href="yourstyle.css">
won't work becuase the Excel file is saving to the user's hard drive.
There's two options:
1) Use the complete url for the stylesheet, such as
<link rel=stylesheet type=text/css href="http://yourserver/yourstyle.css">
or, in case a user later views the file when they don't have a net connection, include (NOT import) the stylesheet, such as
I am a loud man with a very large hat. This means I am in charge