You can include packages for rendering latex fragments - and all latex export from org I guess - along with their options via org-latex-packages-alist. In your case
(add-to-list 'org-latex-packages-alist '("per-mode=fraction" "siunitx" t))
This will most likely not help you with the question being 2 1/2 years old, but maybe someone else comes along looking ...
Checkout the preview-pdf-color-adjust-method option.
Apparently it should use automatically the same foreground color as the buffer text, but at least in my case the problem was that I have only Ghostscript 9.26 installed. As a workaround I downloaded the latest Ghostscript binary and modified preview-gs-command to point to it.