You can define this by yourself. Here is a good example from Arch Linux Forums.
Copy the following code and save it in /usr/share/nano/ as conf.nanorc
# config file highlighting
syntax "conf" "(.(conf|config|cfg|cnf|rc|lst|list|defs|ini|desktop|mime|types|preset|cache|seat|service|htaccess)$|(^|/)(\w*crontab|mirrorlist|group|hosts|passwd|rpc|netconfig|shadow|fstab|inittab|inputrc|protocols|sudoers)$|conf.d/|.config/)"
default text
color magenta "^.*$"
special values
icolor brightblue "(^|\s|=)(default|true|false|on|off|yes|no)(\s|$)"
keys
icolor cyan "^\s(set\s+)?[A-Z0-9_/.%@+-]+\s([:]|>)"
commands
color blue "^\s*set\s+<"
punctuation
color blue "[.]"
numbers
color red "(^|\s|[[/:|<>(){}=,]|])[-+]?0-9*%?($|>)"
keys
icolor cyan "^\s*($if )?([A-Z0-9_/.%@+-]|\s)+="
punctuation
color blue "/"
color brightwhite "(]|[()<>[{},;:=])"
color brightwhite "(^|[|{|:)\s*-(\s|$)"
section headings
icolor brightyellow "^\s([([A-Z0-9_.-]|\s)+])+\s$"
color brightcyan "^\s((Sub)?Section\s(=|>)|End(Sub)?Section\s$)"
color brightcyan "^\s$(end)?if(\s|$)"
URLs
icolor green "\b(([A-Z]+://|www[.])[A-Z0-9/:#?&$=_.-]+)(\b|$| )"
XML-like tags
icolor brightcyan "</?\w+((\s\w+\s=)?\s("[^"]"|'[^']'|!?[A-Z0-9_:/]))(\s*/)?>"
strings
color yellow ""(\.|[^"])"" "'(\.|[^'])'"
comments
color white "#.$"
color blue "^\s##.$"
color white "^;.$"
color white start="<!--" end="-->"
Then include this configuration in /etc/nanorc file as
## Configuration files (catch-all syntax)
include "/usr/share/nano/conf.nanorc"
The first code line in the snippet includes a regular expression that
defines for which file names this syntax highlighting should be used.
Whenever you encounter a config file that is not matched by this, but
you would still like to open it with syntax highlighting, you can
manually select this syntax with nano's -Y switch, like so:
nano -Y conf myConfigFile
Source: nano syntax highlighting: catch-all syntax for configuration files.