bríefly black background when loading site

Hi, there, I have this one pager with a fixed background using some copy-pasted custom code (no coding knowledge). However upon loading the website the background sort of slowly flashes black once and only then loading correctly. Anybody can help me with this?

Here is the custom code I use:

.section-background {
    position: fixed !important;
}
// Anchor Smooth scroll
html {
  scroll-behavior: smooth;
}
a{background-size: 0px !important;
text-decoration: none!important;}

I think it is site background, if you want to change this color, you can use this code to Custom CSS box

body {
	background-color: white !important;
}