style.css

.wp-block-sticky-note-sticky-note {
  background: linear-gradient(rgba(255, 255, 255, 0.1), rgba(0, 0, 0, 0.1));
  box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
  padding: 25px;
  transition: all ease 333ms;
  font-family: "Shadows Into Light Two", Chalkboard, "Comic Sans MS", "Comic Sans", cursive !important;
  font-size: 20px;
  border-radius: 3px;
  max-width: 300px;
  min-height: 200px;
  vertical-align: middle;
  margin: 0 auto !important;
}

.wp-block-sticky-note-sticky-note:hover {
  box-shadow: 0 8px 12px rgba(0, 0, 0, 0.1);
  transform: rotate(1deg);
  transform: scale(1.01) rotate(1deg);
}

.wp-block-sticky-note-sticky-note.is-style-flat {
  position: relative;
  background: #f6e8a5;
  box-shadow: none;
  border-radius: 0;
  border: 1px solid rgba(0, 0, 0, 0.1);

  /* from https://css-tricks.com/notched-boxes/ */
  clip-path: polygon(0% 0%, 100% 0%, 100% calc(100% - 20px), calc(100% - 20px) 100%, 0 100%);
}

/* from https://designshack.net/articles/css/code-a-simple-folded-corner-effect-with-css/ */
.wp-block-sticky-note-sticky-note.is-style-flat::before {
  content: "";
  position: absolute;
  bottom: 0%;
  right: 0%;
  width: 0;
  height: 0;
  transition: all ease 333ms;
  border-top: 20px solid rgba(0, 0, 0, 0.1);
  border-right: 20px solid rgba(0, 0, 0, 0.1);
}

.wp-block-sticky-note-sticky-note.is-style-flat:hover {
  box-shadow: none;
  transform: none;

  clip-path: polygon(0% 0%, 100% 0%, 100% calc(100% - 40px), calc(100% - 40px) 100%, 0 100%);
}

.wp-block-sticky-note-sticky-note.is-style-flat:hover::before {
  border-top: 40px solid rgba(0, 0, 0, 0.1);
  border-right: 40px solid rgba(0, 0, 0, 0.1);

}