SillyTavern/public/scripts/extensions/audio/style.css

.mixer-div {
    display: flex;
    flex-direction: row;
    align-items: center;
    justify-content: center;
    padding: 5px;
}

.audio-mute-button {
    padding: 5px;
    width: 50px;
    height: 30px;
}

.audio-mute-button-muted {
    color: red;
}