mastodon/app/javascript/styles/basics.scss

body {
  font-family: 'mastodon-font-sans-serif', sans-serif;
  background: $ui-base-color;
  background-size: cover;
  background-attachment: fixed;
  font-size: 13px;
  line-height: 18px;
  font-weight: 400;
  color: $primary-text-color;
  padding-bottom: 20px;
  text-rendering: optimizelegibility;
  font-feature-settings: "kern";
  text-size-adjust: none;
  -webkit-tap-highlight-color: rgba(0,0,0,0);
  -webkit-tap-highlight-color: transparent;

  &.system-font {
    // system-ui => standard property (Chrome/Android WebView 56+, Opera 43+, Safari 11+)
    // -apple-system => Safari <11 specific
    // BlinkMacSystemFont => Chrome <56 on macOS specific
    // Segoe UI => Windows 7/8/10
    // Oxygen => KDE
    // Ubuntu => Unity/Ubuntu
    // Cantarell => GNOME
    // Fira Sans => Firefox OS
    // Droid Sans => Older Androids (<4.0)
    // Helvetica Neue => Older macOS <10.11
    // mastodon-font-sans-serif => web-font (Roboto) fallback and newer Androids (>=4.0)
    font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", "Oxygen", "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue", mastodon-font-sans-serif, sans-serif;
  }

  &.app-body {
    position: fixed;
    width: 100%;
    height: 100%;
    padding: 0;
    background: $ui-base-color;
  }

  &.about-body {
    background: darken($ui-base-color, 8%);
    padding-bottom: 0;
  }

  &.tag-body {
    background: darken($ui-base-color, 8%);
    padding-bottom: 0;
  }

  &.embed {
    background: transparent;
    margin: 0;
    padding-bottom: 0;

    .container {
      position: absolute;
      width: 100%;
      height: 100%;
      overflow: hidden;
    }
  }

  &.admin {
    background: darken($ui-base-color, 4%);
    position: fixed;
    width: 100%;
    height: 100%;
    padding: 0;
  }

  &.error {
    position: absolute;
    text-align: center;
    color: $ui-primary-color;
    background: $ui-base-color;
    width: 100%;
    height: 100%;
    padding: 0;
    display: flex;
    justify-content: center;
    align-items: center;

    .dialog {
      vertical-align: middle;
      margin: 20px;

      img {
        display: block;
        max-width: 470px;
        width: 100%;
        height: auto;
        margin-top: -120px;
      }

      h1 {
        font-size: 20px;
        line-height: 28px;
        font-weight: 400;
      }
    }
  }
}

button {
  font-family: inherit;
  cursor: pointer;

  &:focus {
    outline: none;
  }
}

.app-holder {
  &,
  & > div {
    display: flex;
    width: 100%;
    height: 100%;
    align-items: center;
    justify-content: center;
  }
}