less.modifyVars config option doesn't work as promised

When Set options in a global less object before loading the less.js script, less.modifyVars config option doesn't work as promised here:

http://lesscss.org/usage/#using-less-in-the-browser-poll

It will be useful when you define variable default value in a less file and overwrite default in the runtime before the first time parse.

e.g. overwrite the default value using the value in cookie