/usr/share/themes/BlackMATE/gtk-3.0/gtk-main-dark.css is in mate-themes 1.9.2-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | /* The dark colormap is the same as the default one. */
@import url("gtk-main.css");
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