/usr/share/source-highlight/properties.lang is in libsource-highlight-common 3.1.7-1.
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 | # highlighting for (Java) properties files
include "script_comment.lang"
comment start "!"
(type,normal,symbol) = `([^="+]+)([[:blank:]]*)(\+?=)`
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