OMFG, almost all websites have long text gray on white with reduced thickness. This is unreadable with my eyes on my displays.

I personally use Stylus Firefox addon to apply custom styles to webpages. However, you can also put it to chrome/userContent.css according to this article.

Btw. there was a backdoor in Stylish, use Stylus instead.

@namespace url(http://www.w3.org/1999/xhtml);
 
body { font-feature-settings: "liga" 0 !important; }
 
/* firefox reader */
body.light {
  color: #000 !important;
}
 
@-moz-document domain("mestemnakole.cz") {
body {
  color: #000 ;
  background-color: #fff;
}
.blog-comment {
  color: #000;
}
textarea {
  height: 600px !important;
}
}
 
@-moz-document domain("blog.cryptographyengineering.com") {
body {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("blog.erratasec.com") {
body {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("www.copenhagenize.com") {
body {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("web.archive.org") {
#pjgm-content {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("www.itbiz.cz") {
body {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("dinosaurusblog.wordpress.com") {
body {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("www.lesswrong.com") {
.PostsPage-postContent {
    color: #000f !important;
    background-color: #fff;
  }
}
 
@-moz-document domain("blisty.cz") {
p {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("liberland.org") {
.posttext {
    color: #000;
    font-weight: normal;
  }
}
 
@-moz-document domain("nature.com") {
body {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("theguardian.com") {
body {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("www.lekari-bez-hranic.cz") {
body {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("copenhagenize.com") {
.post span {
    color: #000 !important;
    background-color: #fff;
  }
}
 
@-moz-document domain("cyklistesobe.cz") {
#map {
    height: 700px !important;
  }
}
 
@-moz-document domain("practicalcryptography.com") {
body {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("wikipedia.org") {
.mw-body {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("wikibooks.org") {
.mw-body {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("is.muni.cz") {
div.chapter, div.sect1, div.book, div.preface, div.appendix, div.index, div.bibliography, div.glossary {
    height: 800px !important;
  }
}
 
@-moz-document domain("lupa.cz") {
#main {
    width: 95% !important;
  }
}
 
@-moz-document domain("github.com") {
body {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("blog.xkcd.com") {
body {
    color: #000 ;
    background-color: #fff;
    font-weight: normal;
    line-height: 1.2;
  }
}
 
@-moz-document domain("yarchive.net") {
body {
    color: #000 !important;
  }
}
 
@-moz-document domain("www.lupa.cz") {
.urs {
    color: #000 !important;
  }
}
 
@-moz-document domain("solidarita.noblogs.org") {
body {
    color: #000 ;
    background-color: #fff;
    font-weight: normal;
  }
}
 
@-moz-document domain("www.root.cz") {
/*.page { margin-left: 0px !important; max-width: 1290px; }*/
  body { color: #000 !important; }
  /*.sidebar {position: relative !important; }*/
  .content--bg { background: #fff !important; }
  /*.detail__info-bar { display:none !important; }*/
}
 
* {
  animation-iteration-count: 1 !important;
}
 
@-moz-document domain("cryptopals.com") {
body {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("statnice.matfyz.info") {
body {
    color: #000 ;
    background-color: #fff;
  }
}
 
@-moz-document domain("rozpad.cz") {
#logo {
    background-image: none;
    height: 20px;
  }
}
 
@-moz-document domain("wiki.analog.com") {
body {
	  color: #000;
  }
}
 
@-moz-document domain("mojefedora.cz") {
body {
    color: #000 ;
  }
}
 
@-moz-document domain("tutorial.math.lamar.edu") {
img {
    top: 0 !important;
    /*margin-bottom: 50px !important;*/
  }
  .MPScreenEqn {
    position: relative !important;
  }
}
 
@-moz-document domain("a2larm.cz") {
body {
    color: #000 !important;
  }
}
 
@-moz-document domain("slatestarcodex.com") {
/*@-moz-document domain("abclinuxu.cz") {
  .ds_text_user7626 > .ds_text {
    display: none;
  }
}*/
 
#pjgm-content {
    color:#000 !important;
    font: initial !important;
    line-height:1.4 !important;
}
}
 
@-moz-document domain("publi.cz") {
p {color:black !important;}
}