Script Summary: makes the background black and the text white, either in an alternate stylesheet you can select as needed, or immediately
This userscript provides a stylesheet that makes the whole page into white text on black. Depending on the Blacken_immediately setting, that stylesheet is either applied immediately, or (the default) is simply made available as an option you can choose under the browser's "View" menu's "Page Style" submenu.
This userscript has two purposes:
☛ Some web sites, usually thru some inept coding, make some or all of their text quite unreadable because of poor combinations of text color and background color (or background image).
☛ Many people (more than you think!) are "low vision" users -- i.e., they aren't blind, but they have poor eyesight. Making text white-on-black generally makes it much more readable to such readers.