next question. How can I change the header to a background video or a background image?
You can only add an image in the header area from Appearance->Theme Settings->Appearance Options under "Header Display" panel.
Let me know if you need additional help on this.
Are you referring to the slider images? You can change them from Slideshow menu https://prnt.sc/lr7o09 by editing the slide you want and change the featured image (https://www.wpbeginner.com/beginners-guide/how-to-add-featured-image-or-post-thumbnails-in-wordpress/).
The frontpage template is a widgetized area you can change from Appearance->Customize->Widgets->Frontpage Widgets (https://prnt.sc/lr7oqc) while you can use Elementor or any other page builder like described here https://www.cssigniter.com/docs/chords/#how-to-use-the-page-builder-template
Let me know how I can help you more on this.
1.) Slider worked. Any option to use fullscreen videos?
2.) Elementor (home - frontpage) is not working. The elementor is loading in infinity. All other pages are working.
1. No fullscreen videos are supported, but we can install a third party plugin that supports videos (free from WordPress.org) instead of the native one. Its quite easy and I can guide you revert to any slider you choose.
2. The frontpage does not support Elementor because its a widgetized area with no content attached. You can add remove widgets from Appearance->Customize->Widgets->Frontpage Widgets (https://prnt.sc/lr7oqc) while you can use Elementor or any other page builder like described here https://www.cssigniter.com/docs/chords/#how-to-use-the-page-builder-template . Then select this page as your frontpage from Settings->Reading.
If you want to add a content area in the frontpage you can open template-frontpage.php and add
<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
<?php the_content(); ?>
<?php endwhile; endif; ?>
<main id="main" class="home-sections">
Open the default editor and then navigate to the /themes/chords/ folder to find the file.
If you decide to proceed with this modification I would suggest you create a child theme, copy over the files you are editing and make the modifications there. This will allow your changes to survive future theme updates. If you directly edit the theme’s files, all changes will be lost once you update.Have a look at this guide to learn more about child themes and how to create one www.cssigniter.com/ignite/beginners-guide-child-themes/