When viewing your code in the inspector, you can see the following class is added to your elements:
.rd3t-node {
cursor: pointer;
fill: #777;
stroke: #000;
stroke-width: 2;
}
Those stroke attributes are what's causing it. I cannot immediately tell what's causing it, but it seems to be a class added by the react-d3-tree library.
I added the following to your styles.css, which fixed it.
svg .rd3t-node {
stroke-width: 0px;
}
Note that if you add anything that has a stroke to your svg, this might also mess with the stroke. It would be better to check the react d3 tree
documentation to see if you can override it in a better way.