The file you loaded is there, but the link in your html has a problem:
Codice HTML:
<!DOCTYPE html>
<html>
<link rel=”stylesheet” href=”http://ryanlk.altervista.org/custom.css” type=”text/css” media=”screen, projection” />
<head>
<meta charset="UTF-8">
</head>
See those double quotes in the <link> tag? they are not the standard double quote character (") but fancier pairs added possibly by your text editor —compare with those used in the <meta> tag lower down. You should probably use a different software to edit the page, the bare minimum would be something like notepad2.
In addition, the line should be inside the <head> of the page, not above.