Форуми / Cotonti / Skins / Colour Schemes

Most likely you will need loading 2 CSS files at a time. Move all your common CSS rules to a file called e.g. css/base.css and connect it via MY_SKIN.rc.php:

cot_rc_add_file($cfg['themes_dir'].'/'.$usr['theme'].'/css/base.css');

Then put all colour-specific styles to appropriate files like css/blue.css, css/green.css, etc. and connect them the old way in header.tpl:

{HEADER_HEAD}
<link rel="stylesheet" type="text/css" href="{PHP.cfg.themes_dir}/{PHP.theme}/css/{PHP.scheme}.css">

There's a more tricky way using cot_rc_link_file() in MY_SKIN.php, but the above should just work.