Featured Image | WordPress.org

Try the following custom CSS to hide featured image display on individual POSTs and PAGEs:

#masthead.site-header::before {
    background-color: #ffffff;
}
.single .featured-thumbnail, .page .featured-thumbnail {
    display: none;
}

The above code should go into APPEARANCE => CUSTOMIZE => ADDITIONAL CSS.

I can see you already have the CSS below in there. Please remove this CSS code below completely as it does nothing and may introduce errors (the font-size has no selector so it’s completely wrong, and the selector used for the second part does not exist on your site at all).

font-size: 16px;

img.space-bottom-small.space-bottom-xs.wp-post-image {
display:none!important;
}

Thank you so much @gappiah! I did what you told me and it works perfectly!