Jesse Ruderman, a Mozilla developer, runs a site of bookmarklets and small web tools, plus notes on browser security.
HN commenters recommend his bookmarklets, still working in current browsers despite no updates, especially "zap colors", and link his page for making CSS bookmarklets.
Depends on the typical and current conditions of the viewer--however few people realize it. Those that do use f.lux or one of the others.
For example, if the viewer sits in an office under 50 florescent lights, dark on light is best (e.g. Excel-jockies). This viewer "can't imagine any other way!" I often wonder how these people survived before Windows.
If instead they are a digital artist in dim lighting (for better color accuracy) then bright white pages will be blinding, "my eyes are bleeding". Ditto the late-night hacker.
Since I'm online day and night I make judicious use of the negative plugin in compiz (Super+N) and turn down screen brightness at night. When on Windows I use f.lux.
For the browser specifically, the invert-lightness bookmarklet is also helpful when a lot of images are involved:
https://www.squarefree.com/bookmarklets/color.html
For browsers not offering anything comparable to Safari's reader mode, and also for sites whose markup it fails to recognize, I find invaluable the Squarefree "zap" bookmarklet[1], which forces black-on-white and also removes almost any sort of annoyance that gets past Adblock Plus.
I don't know what moves jwz to stick with green on black (yes, very retro chic, but we have colors now! Use them!), but his content (writing, whatever) I find at least occasionally of some value; without this bookmarklet, the nearly instantaneous headache would entirely prevent me reading what he has to say.
[1] https://www.squarefree.com/bookmarklets/zap.html
One of the best things about bookmarklets is that they work on mobiles too.
There's a great collection of bookmarklets at https://www.squarefree.com/bookmarklets/ (Fron the same guy who tracks changes in Firefox's nightly builds: http://www.squarefree.com/burningedge/ )
You can also turn bookmarklets into GreaseMonkey scripts so they can be ran automatically on certain sites (in Firefox): http://www.squarefree.com/2005/05/16/bookmarklets-to-user-scripts/
On Firefox:
1. Go to about:config
2. Right-click anywhere, New -> Boolean:
signon.overrideAutocomplete
Value: true
More info: https://bugzilla.mozilla.org/show_bug.cgi?id=425145
EDIT: Based on the milestone set on that issue, this setting requires Firefox 29. Another workaround is this bookmarklet: https://www.squarefree.com/bookmarklets/forms.html#remember_password referenced from comment https://bugzilla.mozilla.org/show_bug.cgi?id=425145#c16
I don't use 3D view but I've used Jesse Rudderman's topo bookmarklet[1], which is conceptually very similar, for years. It's more useful in IE so you can see what tag IE is failing to close but it's useful for getting a quick feel for unfamiliar markup when doing Greasmonkey hacks and the like.
[1] https://www.squarefree.com/bookmarklets/webdevel.html
For the past 20 years, I've been using the 'zap' and 'zap images' bookmarklets from this web site:
https://www.squarefree.com/bookmarklets/zap.html
They don't work on every site, but they're a good start. 'Reader mode' bookmarklets like https://github.com/lordastley/readable can often be configured for simple white on black text too.