This file is indexed.

/usr/share/themes/BlackMATE/gtk-3.0/gtk-main-dark.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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/* The dark colormap is the same as the default one. */

@import url("gtk-main.css");