No plugin found, so i used a php script. If you want a theme during the day and another theme during the night, paste this code in the header.php in head section after <title> tag
<?php
$hour = date('G');
if (($hour < 8) or ($hour >17)) {
$css = 'style_night.css';
} elseif ($hour > 8) {
$css = 'style_day.css';
}
echo '<link rel="stylesheet" href="/wp-content/themes/default/'.$css.'"type="text/css" media="screen, projection"/>';
?>
This script is using the time from server. If you have some question, just ask.
