How to set context by url argument for node preprocess
Here's a sketch of what you could do, using a custom module:


Define a menu entry for someview/%node.
Define a callback function for the menu entry with a node argument.
Store the node ID for later use. This could be done using drupal_static or the context API.
In your preprocess function, retrieve the node ID and add a CSS class if required.


You could also use Panels to achieve a similar end, I think, but this is a fairly lightweight, if specific solution.

Let me know if you need a fuller explanation. I realise not everyone is familiar with Drupal APIs.

So your menu callback might look like:
function mymodule_callback($node) {
 context_set("mymodule", "active_nid", $node-&gt;nid);
 return views_embed_view((... ...);
}


and in the preprocess:

$active_nid= context_get("mymodule", "active_nid");
 if($active_nid==$node-&gt;nid) {
 ...
}