I want to replace the word (online) or highlight it in green, and I also want to display it on the main advertisements so that a site visitor can see the seller and determine by his logo that he is online. Please write to me in which folder or file can I edit?

It could be in Osclass core or in item.php / item-sidebar.php of your theme

 
The issue has already been resolved!
If anyone is interested, here's what you need to do to make ONLINE look attractive.

To do this you need to replace the line - oc-content/themes/sigma/item-sidebar.php - Line 59 - <?php echo (osc_user_is_online(osc_item_user_id()) ? '<span class="is-online">(' . __('ONLINE', 'sigma') . ')</span>' : ''); ?></p>

Here's a line like this - class="keywords"><?php echo (osc_user_is_online(osc_item_user_id()) ? '<span class="is-online">(' . __('ONLINE', 'sigma') . ')</span>' : ''); ?></p></b>

Be sure to add this code to - oc-content/themes/sigma/css/style.css - .keywords {
  color: #00b906;
}