/usr/share/themes/Radiant-MATE/gtk-3.0/settings.ini 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.
1 2 3 4 | [Settings]
gtk-color-scheme = "base_color:#ffffff\nbg_color:#f6f4f2\ntooltip_bg_color:#000000\nselected_bg_color:#87A752\ntext_color:#3C3C3C\nfg_color:#4c4c4c\ntooltip_fg_color:#ffffff\nselected_fg_color:#ffffff\nlink_color:#A7BB85\nbg_color_dark:#3c3b37\nfg_color_dark:#dfdbd2"
gtk-auto-mnemonics = 1
gtk-alternative-sort-arrows = true
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