Does anyone know of an extension for firefox (or chrome) that replaces white backgrounds (or even better all light colours) with darker shades?

Flux and co are all well and good, but large areas of white (even when orange-ish) are still a strain, and it seems fairly simple issue to solve (at least in the most simple case; replace #ffffff backgrounds).

Also as a tip for anyone with a windows laptop; you may find, like me, that linux allows your screen to be set much darker than the sparse intervals in windows allow (maybe there is a windows program that allows fine brightness setting?). Macbooks seem to have a very low brigtness setting, although i can't say i've used one in the dark.