2020 update: just use bat(1) instead!!!! bat has git integration and also can show invisible characters! OLD STUFF: How to enable syntax-highlighting in less. Use `less -N` (or type -N while in less) to enable line numbers. Based on the procedure described in http://superuser.com/questions/71588

centos-install-syntax-highlighting-in-less.sh

# Enable syntax-highlighting in less.
# Last tested on CentOS 6.3.
#
# First, add these two lines to ~/.bashrc
#     export LESSOPEN="| /usr/bin/src-hilite-lesspipe.sh %s"
#     export LESS=" -R "

sudo yum -y install boost boost-devel ctags

wget http://springdale.math.ias.edu/data/puias/unsupported/6/x86_64/source-highlight-3.1.6-3.puias6.x86_64.rpm
source-highlight-3.1.6-3.puias6.x86_64.rpm
sudo yum -y install source-highlight

macports-install-syntax-highlighting-in-less.sh

# Enable syntax-highlighting in less.
#
# First, add these two lines to ~/.bashrc
#     export LESSOPEN="| /opt/local/bin/src-hilite-lesspipe.sh %s"
#     export LESS=" -R "

sudo port install source-highlight

On fish shell what worked for me was:

$ brew install source-highlight
$ vim ~/.config/fish/functions/less.fish add the following content

function less
  src-hilite-lesspipe.sh $argv | less -N
end

$ source ~/.config/fish/config.fish
$ less

For dark displays use the esc256 style in src-hilite-lesspipe.sh:

#! /bin/bash

for source in "$@"; do
    case $source in
	*ChangeLog|*changelog) 
        source-highlight --failsafe -f esc256 --lang-def=changelog.lang --style-file=esc256.style -i "$source" ;;
	*Makefile|*makefile) 
        source-highlight --failsafe -f esc256 --lang-def=makefile.lang --style-file=esc256.style -i "$source" ;;
	*.tar|*.tgz|*.gz|*.bz2|*.xz)
        lesspipe "$source" ;;
        *) source-highlight --failsafe --infer-lang -f esc256 --style-file=esc256.style -i "$source" ;;
    esac
done

( sed 's,esc,esc256,g' will help ;)

2020 update I think that bat(1) replaces all of this and that one should use bat going forward instead of the less / source-highlight combo. https://github.com/sharkdp/bat#syntax-highlighting