Layering text on top of video in wordpress

If you can do it in html then you can do it in WordPress.
At the very least you could create a custom page template that has your html wired directly into it.
More likely you can use shortcodes to inject your code into your pages, details on shortcodes here:
https://codex.www.ads-software.com/Shortcode_API
You could put your functions into either a plugin or your functions.php file of your theme.

To edit your functions.php and/or add custom css you should be working in a child theme, details here:
creating a child theme https://codex.www.ads-software.com/Child_Themes