HTMLS CSS Rendering Issue

Hi, hope you doing well. I need some help regarding the html css and layer concept. Actually, im applying the html css code in different sections with multiple colors. All things are working perfectly but one thing is missing which is that the model box is not appearing. It feels like it’s behind the HTML screen.

So my question is how can I render my box in screen with HTML full screen so I can see my model while scrolling. Your help would be highly appreciated for me thanks