Writing custom theme

Hi,

Following http://www.branchable.com/tips/customizing_the_look_of_your_site/ ,

I would like to completely replace the default style.css so that I can write my custom style and my custom template. But your documentation says to not replace style.css (and when I try to do this, the site says 'Failed!').

Connection log shows this:

Request URL: http://guppy.branchable.com/ikiwiki.cgi
Method: POST
Code: HTTP/1.1 200 OK

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <base href="/" />
    <meta charset="utf-8" /> 
    <title>Error</title>
    <link rel="icon" href="/favicon.ico" type="image/x-icon" /> 
    <link rel="stylesheet" href="/style.css" type="text/css" /> 
    <link rel="stylesheet" href="/local.css" type="text/css" /> 
</head>
<body> 
    <article class="page"> 
        <section class="pageheader">    
            <header class="header">
                <span>
                    <span class="parentlinks">
                        <a href="/">guppy</a>/
                    </span>
                    <span class="title"> Error </span>
                </span>
            </header>
            <nav class="actions"> 
                <ul>
                    <li><a href="/ikiwiki.cgi?do=prefs">Preferences</a></li>
                    <li><a href="/ikiwiki.cgi?do=branchable">Branchable</a></li>
                </ul>
            </nav>
        </section>
        <div id="pagebody"> 
            <section id="content" role="main">
                <p class="error">Error: CGI::tmpFileName failed to return the uploaded file name </p>
            </section>
        </div> 
        <footer id="footer" class="pagefooter" role="contentinfo"> <!-- from guppy --> </footer>
    </article>
</body>
</html>

And I get the same error message even when I try to upload local.css -- odd, as it was designed to be customizable!

Related discussion: https://ikiwiki.info/forum/Error:_CGI::tmpFileName_failed_to_return_the_uploaded_file_name/ I tried to make use of it, but I don't know what ikiwiki version we have at Branchable.

How do I best achieve what I want?

Regards,
Svetlana.