[ESS] "ess-mode-font-lock-keywords" variable?

[Vitalie Spinu]

Dear Ess Community,

The ESS documentation (12.1 Syntactic highlighting of buffers) refers to  
"ess-mode-font-lock-keywords" variable for customization of syntax  
highlighting keywords.

Unfortunately I could not find it nor through M-x apropos-variable RET  
ess-mode-font-lock-keywords
nor in customisation buffers in emacs.

Can anybody refer me to the place were these keyword are defined?

Basically I would like to have a  bit more colour in my files (similarly  
to R-wiki). May be somebody has already enhanced his/her syntax  
highlighting and can share the experience. Would appreciate it very much.

Many,many thanks ,
Vitalie.