[css] How to apply a CSS filter to a background image

If you want to content to be scrollable, set the position of the content to absolute:

content {
   position: absolute;
   ...
}

I don't know if this was just for me, but if not that's the fix!

Also since the background is fixed, it means you have a "parallax" effect! So now, not only did this person teach you how to make a blurry background, but it is also a parallax background effect!