How to make CSS visible only for Opera

The only way I can think of is to check the user agent and only reference the style sheet when it's an opera browser. Since the user agent can be messed with this might not be 100% reliable.

You can use javascript to write out a <link> to include a specific CSS file.

if (navigator.userAgent.indexOf(’Opera’) != -1) {
    document.write(””);
}
else {
    document.write(””);
}

For Opera 7 you can use this:

/*Visible to only Opera*/
@media all and (min-width: 0) {
    /* css rules here */
}

However, it's generally bad practice to do styling based on browser-sniffing.