Hi,

I am trying to make a Delta Child_Theme with the following Index.php information:

<?php
/*
Theme Name: Delta CHILD Osclass Theme
Theme URI: https://osclasspoint.com/osclass-themes/general/delta-osclass-theme_i170
Description: Premium front-end theme for classifieds script Osclass
Version: 1.5.5
Author: MB Themes
Author URI: https://osclasspoint.com
Widgets: header,footer
Theme update URI: delta-osclass-theme
Product Key: L3IN81p4BJP0UU4pc0Xk
Parent Theme: delta
*/
?>

When I go to active the Delta_Child Theme in Appearance - Manage Theme.
When I activate it, the theme that appears is the Sigma Theme - Picture attached

Sigma may be shown as gui backup in case front theme failed.

I thought that when activating the Delta child_theme I would get a copy similar to the Delta parent theme. However, what I get is the Sigma theme. Delta theme and Sigma theme have a completely different layout. I want to make changes to the Delta theme and not the Sigma theme. Attached are pictures of both themes

Correct, but if you do it incorrectly, to avoid front failure, it use gui.
To confirm this, check path in source code.

Where can I check the path in source code?

page source code.
you should have paths oc-content/themes/delta_child/...

in case of failure, it will be oc-includes/osclass/gui/...

I am testing the software in Xampp: Localhost

This is the Path of the Delta parent theme
Xampp_7_4_33/htdocs/osclass2/oc-content/themes/Delta

This is the Path of the Delta_Child theme
Xampp_7_4_33/htdocs/osclass2/oc-content/themes/Delta_Child/

Theme folder can only be lowercase.

Now is working. Thank you for being so helpful.

Welcome