JSAでやろうと思ってたが
bookmarkletでつくってみた。

bookmarklet darkens

javascript:(function(){var%20newSS,%20styles='*%20{%20background:%20#333333%20!%20important;%20color:%20#cccccc%20!important%20}%20:link,%20:link%20*%20
{%20color:%20#4682b4%20!important%20}%20:visited,%20:visited%20*%20{%20color:%20#4682b4%20!important%20}';%20if(document.createStyleSheet)%20
{%20document.createStyleSheet("javascript:'"+styles+"'");%20}%20else%20{%20newSS=document.createElement('link');%20newSS.rel='stylesheet';%20
newSS.href='data:text/css,'+escape(styles);%20document.documentElement.childNodes[0].appendChild(newSS);%20}%20})();