Just for the sake of argument:
Code php:
header('Content-type: text/css');
 
$css = file_get_contents('screen.css');
 
$variable_array = array(
 'font_header' => 'arial, helvetica, sans-serif',
 'font_text' => 'georgia, times, serif',
 'color_header_background' => '#000',
 'color_header_text' => '#fff'
);
 
foreach($variable_array as $key=>$value) {
 $search = '{var:'.$key.'}';
 $css = str_ireplace($search, $value, $css);
}
 
echo $css;

Code css:
html {
 font-family: {var:font_text};
}
 
h1 {
 background-color: {var:color_header_background};
 color: {var:color_header_text};
 font-family: {var:font_header};
}
 
h2 {
 background-color: {var:color_header_background};
 color: {var:color_header_text};
 font-family: {var:font_header};
}
 
th {
 background-color: {var:color_header_background};
 color: {var:color_header_text};
 font-family: {var:font_header};
}