zotero/scss/components/_titleBar.scss

// Styling for displaying tabs in the title bar
:root:not([legacytoolbar="true"]) {
	--tab-min-height: 36px;

	& {
		--autocomplete-popup-background: -moz-field;
		--autocomplete-popup-color: -moz-fieldtext;
		--autocomplete-popup-border-color: ThreeDShadow;
		--autocomplete-popup-highlight-background: Highlight;
		--autocomplete-popup-highlight-color: HighlightText;
		/* Note: Setting this to 0 (without px) breaks CSS calculations for OSX. */
		--space-above-tabbar: 0px;
	}
	
	&:-moz-window-inactive {
		--toolbar-bgcolor: -moz-mac-chrome-inactive;
	}

	/* ::::: Tabs in Titlebar :::::: */
	
	&:not([chromemargin]) > #titlebar {
		display: none;
	}
	
	/**
	 * For tabs in titlebar on OS X, we stretch the titlebar down so that the
	 * tabstrip can overlap it.
	 */
	&[tabsintitlebar] > #titlebar {
		min-height: calc(var(--tab-min-height) + var(--space-above-tabbar));
	}
	
	&:not([tabsintitlebar]) .titlebar-placeholder {
		visibility: collapse;
	}
	
	@media (-moz-platform: macos) {
		/* NB: these would be margin-inline-start/end if it wasn't for the fact that OS X
		* doesn't reverse the order of the items in the titlebar in RTL mode. */
		.titlebar-placeholder[type="caption-buttons"],
		.titlebar-buttonbox {
			margin-right: 12px;
			margin-left: 12px;
		}
		
		.titlebar-buttonbox {
			-moz-appearance: -moz-window-button-box;
		}

		.titlebar-icon {
			display: none;
		}
	}
}

:root:not([sizemode=maximized], [sizemode=fullscreen]) .titlebar-restore,
:root:is([sizemode=maximized], [sizemode=fullscreen]) .titlebar-max {
	display: none;
}

#zotero-title-bar {
	border-bottom: var(--material-panedivider);
}

#titlebar {
	display: flex;
	justify-content: center;
	-moz-window-dragging: drag;
	menu {
		font-size: var(--zotero-font-size);
	}
}