Line between featured image and post

In the “Additional CSS” section of the dashboard add this:

.colors-dark .site-header,
.colors-dark .single-featured-image-header {
    background: #000;
}

https://codex.wordpress.org/CSS#Custom_CSS_in_WordPress

Add this as well:

.colors-dark .single-featured-image-header, 
.colors-dark .navigation-top,
.colors-dark .main-navigation li, 
.colors-dark .entry-footer, 
.colors-dark #comments {
    border-color: #000;
}