/* Remove black background from search button */
.wp-block-search .wp-block-search__button {
  background: transparent !important; /* no background */
  border: none !important;            /* no border */
  padding: 0 !important;              /* tighter spacing */
}

/* Make sure icon color stays visible */
.wp-block-search .wp-block-search__button svg {
  fill: #000; /* set icon color you like */
  width: 20px; /* adjust size */
  height: 20px;
}