zotero/scss/components/_selectItemsDialog.scss

#select-items-dialog {
    padding: 2em;

    #zotero-select-items-container {
        display: flex;
        flex-direction: column;
        gap: 8px;
    }

    #zotero-collections-tree-container {
        min-width: 200px;
    }

    #search-toolbar {
        display: flex;
        flex-direction: row;
        justify-content: flex-end;
    }

    #search-toolbar {
        flex: 0 0 auto;
    }

    #collections-items-container {
        flex: 1 1 auto;
        margin-bottom: 8px;
    }
}