/usr/share/plank/themes/Matte/dock.theme is in libplank-common 0.11.4-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | [PlankTheme]
[PlankDockTheme]
HorizPadding=1
TopPadding=1.5
BottomPadding=2
IconShadowSize=0
CascadeHide=false
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