/usr/share/gthumb/contactsheetthemes/rainbow.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=Rainbow
[Background]
Type=full
Color1=#1212c0c02f2f
Color2=#ffff00000000
Color3=#00000000ffff
Color4=#d2f6d2f610b5
[Frame]
Style=simple-with-shadow
Color=#ffffffffffff
[Header]
Font=Sans 22
Color=#ffffffffffff
[Footer]
Font=Sans Bold 12
Color=#ffffffffffff
[Caption]
Font=Sans 8
Color=#ffffffffffff
|