This file is indexed.

/usr/share/themes/Radiant-MATE/gtk-3.0/gtk-widgets-assets-radiance.css is in ubuntu-mate-themes 18.04.11.

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 check {
    -gtk-icon-source: url("assets/check-menuitem-unselected.png");
}

menu radio {
    -gtk-icon-source: url("assets/check-menuitem-unselected.png");
}

menu check:checked {
    -gtk-icon-source: url("assets/check-menuitem-dark.png");
}

menu check:checked:disabled {
    -gtk-icon-source: url("assets/check-menuitem-insensitive-dark.png");
}

menu radio:checked {
    -gtk-icon-source: url("assets/radio-menuitem-dark.png");
}

menu radio:checked:disabled {
    -gtk-icon-source: url("assets/radio-menuitem-insensitive-dark.png");
}