Are you using a plugin or some type of css editor? I can see this when I inspect your code –
body {
background: rgb(246, 248, 248);
font-size: 100%;
padding: 3em 0;
}
….but it’s not coming from the style.css of the theme.
The border is actually 3em of padding….so if you remove the padding code above it will instantly remove it. Since it’s not there by default I assume there must be a plugin or some type of editor adding this code in.