Add the following code to your obsidian.css file located in your directory/vault folder with all your .md files. (If you don’t have one, make one). Change the #161616 and #a973ff to whatever colors you’d like. Let us know if you need more explanation, we’re all happy to help 
/* Dot color */
.graph-view.color-fill {
color: #a973ff;
}
/* Dot border color */
.graph-view.color-circle {
color: #a973ff;
}
/* Dot color upon hover */
.graph-view.color-fill-highlight {
color: #a973ff;
}
/* Line color */
.graph-view.color-line-highlight {
color: #a973ff;
}
/* Background color */
canvas {
background-color: #161616;
background-blend-mode: soft-light;
}
