As an example, my current Userstyle for it, minus other unrelated bits.

Code:

/* Color Variables */
:root {
--greenVibrant: #1c8d4f;
--greenLight: #bad6c2;
--greenDark1: #0a1c0b;
--greenDark2: #151f14;
}

/* Quick Reply Box */
.quickReply > .messageUserInfo {
display: none;
}

#QuickReply {
padding: 10px;
margin-left: 0;
min-height: 100px;
}

.redactor_toolbar li.redactor_btn_group ul {
border-right: 0;
}

.redactor_toolbar {
background-color: var(--greenDark2);
border-bottom: 1px solid black;
}

.redactor_toolbar li a:hover {
color: var(--greenVibrant);
}

.button {
background-color: var(--greenDark2);
}