Politically speaking, right now the White Court can disavow Thomas safely. So, the svartalves are allowed to kill him under the Accords... if they could find him. Marcone had custody of him last, so the svartalves might have a legitimate beef with him, but there are probably monetary options for smoothing that over in the Accords. To go after Harry overtly, they'd need to prove that Harry stole Thomas.
Harry definitely burned his bridges with the svartalves, but I don't think we're looking at open war right now. The short stories post BG didn't seem like it either.