My latest Atom style sheet ...

styles.less

.indent-guide-improved {
  background-color: fade(white, 5%);
  &.indent-guide-stack {
    background-color: fade(cyan, 20%);
    &.indent-guide-active {
      background-color: fade(yellow, 20%);
    }
  }
}

/* Test comment. */
atom-text-editor::shadow {
  .selection .region {
    box-shadow: inset 0 0 1px 1px fade(#fff, 50%);
    border-radius: 3px;
    background: linear-gradient(
      to right,
      fade(#ff0, 20%) 0%,
      fade(#0f0, 20%) 15%,
      fade(#0ff, 20%) 30%,
      fade(#00f, 20%) 50%,
      fade(#f0f, 20%) 65%,
      fade(#f00, 20%) 80%,
      fade(#ff0, 20%) 100%
    );
    background-size: 500px;
  }
}