/usr/share/themes/Greybird/gtk-3.0/settings.ini is in shimmer-themes 1.3.0-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | [Settings]
gtk-color-scheme = "base_color:#fcfcfc\nbg_color:#CECECE\ntooltip_bg_color:#000000\nselected_bg_color:#9db6cd\ntext_color:#000000\nfg_color:#3c3c3c\ntooltip_fg_color:#FFFFFF\nselected_fg_color:#F5F5F5\nlink_color:#74a0c6\nbg_color_dark:#686868\nfg_color_dark:#fcfcfc"
gtk-auto-mnemonics = 1
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