gnusocial/public/plugins/AttachmentCollections/assets/css/widget.css

.attachment-collections {
    background-color: red;
}
.attachment-collections .attachment-collections-selections-options {
    display: flex;
    margin-top: 12px;
    align-items: center;
    justify-content: space-between;
}
.attachment-collections .attachment-collections-selections-options button {
    margin: 0 !important;
}
#add_collections {
    height: auto !important;
    max-height: 20rem !important;
    overflow-y: auto !important;
}