Joomla - Put a full-page type slider into the background

If your template/theme doesn’t have anything else in there, then you could put your slider into the background to be in a fixed position behind everything with this code, just put it right after the <body> part of your website:
probably into the index.php file of the template

<div style="position:fixed;top:0;bottom:0;left:0;right:0;z-index:-1;">
<jdoc:include type="modules" name="bgsliderposition" />

Troubleshooting if the slider is there, just your template/theme hides it: give a bigger (positive) z-index to it. If the slider is there, just something hides it, you could try to modify your template/theme like this.