/usr/share/gthumb/contactsheetthemes/default.cst is in gthumb-data 3:3.3.1.is.3.2.7-0ubuntu1.
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 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 | [Theme]
Name=Default
[Background]
Type=full
Color1=#e0d3c0
Color2=#b1c3ad
Color3=#e8e8ea
Color4=#bad8d8
[Frame]
Style=simple-with-shadow
Color=#94d6Cd
[Header]
Font=Arial 22
Color=#D5504A
[Footer]
Font=Arial Bold 12
Color=#394083
[Caption]
Font=Arial 10
Color=#414141
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