zotero/chrome/content/zotero-platform/win/overlay.css

:root {
	--theme-border-color: #cecece;
}

/* Hide horrible blue effect for menu bar and toolbar */
#navigator-toolbox {
	-moz-appearance: none;
}

#zotero-collections-splitter:not([state=collapsed]),
#zotero-items-splitter:not([state=collapsed]),
#zotero-tags-splitter:not([state=collapsed]),
#zotero-context-splitter:not([state=collapsed]),
#zotero-context-splitter-stacked:not([state=collapsed]) {
	border: 0;
	background-color: transparent;
	position: relative;
	/* Positive z-index positions the splitter on top of its siblings and makes
	it clickable on both sides. */
	z-index: 1;
}

#zotero-context-splitter:not([state=collapsed]),
#zotero-context-splitter-stacked:not([state=collapsed]) {
	z-index: 0;
}

#zotero-collections-splitter:not([state=collapsed]),
#zotero-items-splitter:not([state=collapsed]):not([orient=vertical]),
#zotero-tags-splitter:not([state=collapsed]),
#zotero-context-splitter:not([state=collapsed]) {
	border-inline-end: 1px solid var(--theme-border-color);
	min-width: 0;
	width: 3px;
	margin-inline-start: -3px;
}

#zotero-tags-splitter:not([state=collapsed]),
#zotero-items-splitter:not([state=collapsed])[orient=vertical],
#zotero-context-splitter-stacked:not([state=collapsed]) {
	border-block-end: 1px solid var(--theme-border-color);
	min-height: 0;
	height: 3px;
	margin-block-start: -3px;
}

#zotero-collections-splitter > grippy,
#zotero-items-splitter > grippy,
#zotero-tags-splitter > grippy,
#zotero-context-splitter > grippy {
	border: 0;
}

#zotero-collections-splitter:not([state=collapsed]) > grippy,
#zotero-items-splitter:not([state=collapsed]) > grippy,
#zotero-tags-splitter:not([state=collapsed]) > grippy,
#zotero-context-splitter:not([state=collapsed]) > grippy,
#zotero-context-splitter-stacked:not([state=collapsed]) > grippy {
	display: none;
}

tree {
	border-width: 0;
}

#zotero-tag-selector groupbox {
	-moz-appearance: none;
	padding: 0;
	border: 0;
}

#tags-box {
	padding-top: 0.1em;
	padding-left: 0.05em;
}

#zotero-item-pane-groupbox {
	-moz-appearance: none !important;
	border-width: 0;
}

.zotero-editpane-item-box > scrollbox, .zotero-view-item > tabpanel > vbox, 
#zotero-editpane-tags > scrollbox, .zotero-editpane-related {
	padding-top: 5px;
}

#zotero-editpane-tags > scrollbox {
	padding-left: 5px;
}

.zotero-view-item > tabpanel > vbox {
	padding-left: 5px;
}