I browse this website quite often for fun making build lists and looking at parts and the such.
The near-entirely white background really hurts my eyes despite my brightness at it's lowest.. :(
Dark mode is coming after the responsive update. No official ETA at this time.
I just found out a site where you can download an extension, where you also can customize your dark theme on pcpartpicker.
Here is the link
Well, since manirelli said that there is an update incoming, the only thing that remains, is to wait.