Style.css line 1372 is this:
.paging-navigation .nav-previous,
.paging-navigation .nav-next,
.post-navigation .nav-previous,
.post-navigation .nav-next,
.image-navigation .previous-image,
.image-navigation .next-image {
display: table-cell;
vertical-align: middle;
width: 2em;
height: 2em;
background-color: #9ca9b0;
border-radius: 50%;
color: #fff;
text-align: center;
padding-top: 4px;
font-size: 1.250em;
}
For the posts, you would need to do custom css to override it with your own colour choice for the background-color and the font icon (arrow) color.
.post-navigation .nav-previous,
.post-navigation .nav-next {
background-color: #9ca9b0;
color: #fff;
}
Make sure you are using a child theme for this (which this theme has one you can install), or for just CSS changes, use a plugin for editing CSS. If you use Jetpack, this has the built in Edit CSS plugin you can use.