Hi , which section of the one pager to be more exact?

Hi Angela

you already have these instructions in custom css that sets the background to light grey, that's why it does not change:

.page-template-one-page section#blog {
    background-color: #f5f5f5 !important;
}

.page-template-one-page #blog .container {

    margin-top: 0px;
    top: 0px;
    padding: 100px 20px;
    background-color: #f5f5f5;
}