/usr/share/themes/Greybird/lightdm/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:#686868\nbg_color:#444444\ntooltip_bg_color:#fffde0\nselected_bg_color:#9db6cd\ntext_color:#efefef\nfg_color:#efefef\ntooltip_fg_color:#333333\nselected_fg_color:#F5F5F5\nlink_color:#74a0c6\nbg_color_dark:#686868\nfg_color_dark:#fcfcfc"
gtk-auto-mnemonics = 1
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