custom.css

/* Remove the slightly darker background on hover over messages. */
.theme-dark {
  --background-message-hover: unset;
}

/* Remove all emoji bar buttons except for the emoji picker itself. */
div[class*='buttonContainer-'] div[class*='emojiButton-'],
div[class*='buttonContainer-'] div[class*='emojiSeparator-'],
div[class*='buttonContainer-'] div[class*='button-']:last-child {
  display: none;
}

/* Remove emoji picker border. */
div[class*='buttonContainer-'] div[class*='wrapper-'] {
  box-shadow: none;
}

/* Resize and place emoji picker within message height, positioned at the top. */
div[class*='buttonContainer-'] div[class*='buttons-'] {
  top: 0;
}
div[class*='message-'][class*='groupStart-'] div[class*='buttonContainer-'] div[class*='buttons-'] {
  top: 22px;
}
div[class*='buttonContainer-'] div[class*='wrapper-'] {
  height: 26px;
}
div[class*='buttonContainer-'] div[class*='button-']:nth-last-child(2) {
  height: 18px;
}