Custom CSS

Worked it out, although I find no actual documentation about the supported classes:

/* bbPress Topic Attachment */
#bbpress-forums .bbp-attachments-form  {
	font-size: 12pt;
}
/* bbPress Topic Attachment */
#bbpress-forums .bbp-attachments-form input {
	border: 1px solid #79c777;
	background-color: #5d6163;
	color: black;
	border-radius: 4px;
	width: 100%;
	padding: 10px;
}