Passing URL variables

Hi!

Sorry to ask that, because it seems it has been solved in some way, but the passing variables through URL is driving me cray.

I just want to display some products based on which link has the user clicked. To do that, I want to pass a parameter called “theme”, so the URL query gets like that:

https://www.ticsipaisatge.cat/?page_id=604?tema=Platoniq

I’ve made a new function on my functions.php like this:

function add_query_vars($aVars) {
    $aVars[] = "theme";    // represents the name of the theme
    return $aVars;
}

add_filter('query_vars', 'add_query_vars');

And I’m trying to retrieve it by doing this on the end page:

if(isset($wp_query->query_vars['theme'])) {
    $theme = urldecode($wp_query->query_vars['theme']);
}

echo $theme;

But nothing is “echoed”. I feel like I’m doing everything ok but I’m missing something. Any help?

Thank you in advance,
Hector