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/usr/share/themes/BlueMenta/gtk-3.0/gtk-fallback.css is in mate-themes 3.22.16-4ubuntu1.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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menu {
	border-image: linear-gradient(to top,
		alpha(@borders, 0.80),
		alpha(@borders, 0.60) 33%,
		alpha(@borders, 0.50) 66%,
		alpha(@borders, 0.15)) 1 1 1 1/ 1px 1px 1px 1px;
}

menubar menu {
	border-image: linear-gradient(to top,
		alpha(@borders, 0.80),
		alpha(@borders, 0.60) 33%,
		alpha(@borders, 0.50) 66%,
		transparent 99%) 1 1 1 1/ 1px 1px 1px 1px;
}