You have this CSS on your site:
.inside-article, .sidebar .widget, .comments-area {
border: 1px solid #e8eaed;
box-shadow: 0 0 10px rgba(232,234,237,.5);
}
Delete this CSS to remove the border. I cannot see where the CSS is coming from because of you have a cache enabled, but its most likely in Customizer > Additional CSS
For the additional space above the post, you can increase the Top Padding in Customizer > Layout > Container. If you want it to only apply to the single post then use this CSS:
.single #page .inside-article {
padding-top: 40px;
}