You can use custom CSS to change that. This blog maybe help you
https://www.joomshaper.com/blog/which-line-of-css-i-have-to-edit-code-inspector-in-action
You can use this custom CSS
.sp-megamenu-parent > li.active > a, .sp-megamenu-parent > li:hover > a {
background: #292D2D !important;
}
.sp-page-builder .page-content #section-id-1499755063964{
background: #292D2D !important;
}
#sp-footer {
background: #292D2D !important;
}
Path towards custom CSS: Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.