If Ribbon Lite doesn’t have within its configuration settings an option to completely hide the sidebar (e.g. under “Appearance > Customize”), you’re likely going to have to modify some of the theme files themselves (PHP and likely CSS as well).
Removing the WordPress sidebar is covered in detail in numerous places all over the internet; it’s going to be a similar process regardless of the theme you’re using:
For example, in a typical WordPress theme you may need to edit
index.php, page.php, single.php, archive.php, home.php and so on.Open a template file to edit it and then locate the line that looks
like this:<?php get_sidebar(); ?>Delete the line that represents the sidebar that you don’t want to
display.Now, save and upload the file back to your website. Repeat the process
for all template files responsible for displaying different pages on
your website.
Taken from: http://www.wpbeginner.com/wp-themes/how-to-remove-the-sidebar-in-wordpress/
Ideally you’ll do this in a child theme, that way your changes don’t get overwritten if the Ribbon Lite theme ever receives an update.