You could change the method "_handle_keywords" in lib/plugins/tag/action.php according the following code:
function _handle_keywords(&$data) {
global $ID;
// Fetch tags for the page; stop proceeding when no tags specified
$tags = p_get_metadata($ID, 'subject', METADATA_DONT_RENDER);
if(is_null($tags)) true;
// Replace underscores with blanks
foreach($data->data['meta'] as &$meta) {
if($meta['name'] == 'keywords') {
$meta['content'] = str_replace('_', ' ', $meta['content']);
$t = explode(',', $meta['content']);
if(in_array('deprecated', $t)) {
$data->data['style'][] = array('type' => 'text/css',
'_data' => '.dokuwiki .page {background-color: red;}');
}
}
}
}