You can change the color code in the following CSS and then add it in the “Appearance => Customize => Additional CSS” box:

.wp-block-file .wp-block-file__button:hover,
.wp-block-file .wp-block-file__button:focus,
.wp-block-button .wp-block-button__link:hover,
.wp-block-button .wp-block-button__link:focus {
  	background-color: #ef3636;
}