The slider border is coming from the slider, you may have to change the skin or create custom css to override it.
Look around line 2755 to change the background color to #000 –
.epik-black .head-wrap,
.epik-black .welcome,
.epik-black .home-feature-bg-dark,
.epik-black .author-box,
.epik-black .footer-widgets {
background: #111;
}