I've noticed this code when I viewed page source:
<link rel="stylesheet" href="css:style.css" />
It doesn't make sense to me, 'cos it's NOT a valid URL to an external CSS file. The developer said something like this:
"It is reserved for the file > css/custom.css"
They also said:
The missing URL for the custom.css depends on the php code to include the css file.
If there is no css file to load, the code is blank"
I found this line of PHP code in template.config.php