zotero/scss/elements/_duplicatesMergePane.scss

duplicates-merge-pane {
	&, #zotero-duplicates-merge-version-select, #zotero-duplicates-merge-item-box-container {
		display: flex;
		flex-direction: column;
		gap: 8px;
	}

	padding-top: 9px;

	> groupbox {
		// Override default margin/padding that breaks our styles here
		margin: 0;
		padding: 0;
		padding-inline: 8px;
		min-height: 0;

		> :is(description, label, button) {
			margin: 0;
		}
	}

	#zotero-duplicates-merge-field-select {
		margin-bottom: 9px;
	}

	#zotero-duplicates-merge-item-box-container {
		flex: 1;
		padding-inline: 8px;
		overflow-y: scroll;
		border-top: var(--material-border-quinary);

		collapsible-section > .head {
			display: none;
		}
	}

	/* Show duplicates date list item as selected even when not focused
	(default behavior on other platforms) */
	#zotero-duplicates-merge-original-date:not(:focus) > richlistitem[selected="true"] {
		background-color: -moz-cellhighlight;
		color: -moz-cellhighlighttext;
	}
}