https://forum.amane.moe/ で default-dark/theme.css に追加して使用しているCSS (under GNU Affero General Public License v3.0)

custom.css

.h-entry {
  min-width: 80%;
  min-height: 0;
}
.h-entry main {
  margin: 0 20px;
}
.card h3 {
  margin: 20px;
}
main h1.p-name,
main h2.p-summary,
main h3.p-name {
  font-size: 1em;
}
main .article-info {
  margin: 0 auto 0;
}
main header.article > div:not(.shadow) {
  margin: 2em auto 0;
}
@media screen and (max-width: 600px) {
  main article {
    margin: 0 .5em 2.5em;
  }
}
@media screen and (min-width: 600px) {
  main article {
    margin: 0 auto 2.5em;
  }
}
main .article-meta, main .article-meta button {
  margin-top: 0;
}
main .article-meta .split .right {
  font-size: 0.8em;
}
body {
  font-family: initial;
}