WordPress投稿エディタでアップロードする画像の必須スタイルのCSSサンプルです。

wp-posts-image.css

/* -------------------------------------------------------------------
# WordPress required image style
------------------------------------------------------------------- */
/*
## image align

margin is set by $font-size base
*/
.aligncenter {
  display: block;
  margin: 0.5rem auto 2rem;
}

.alignright {
  display: block;
  float: right;
  margin: 0.5rem 0 2rem 2rem;
}
.alignleft {
  display: block;
  float: left;
  margin: 0.5rem 2rem 2rem 0;
}
.wp-caption {
  margin-bottom: 2rem;
  max-width: 100%;
}
.wp-caption-text {
  margin: 0.5rem 0 !important;
  padding: $font-size/4 !important;
  font-size: 0.875rem;
  line-height: 1.375rem;
}