I tried this :
[code]var categorie = [
{ key: ‘CAF’, label: ‘CAF’, css: ‘color:#ffbfff;’ },
{ key: ‘Prévention’, label: ‘Prévention’, css: ‘color:#7fbfff;’ },
{ key: ‘DV’, label: ‘DV’, css: ‘color:#00bf3f;’ },
{ key: ‘PRE’, label: ‘PRE’, css: ‘color:#7f00bf;’ },
{ key: ‘Réunion_équipe’, label: ‘Réunion_équipe’, css: ‘color:#ffff00;’ },
{ key: ‘Formation’, label: ‘Formation’, css: ‘color:#ffbf00;’ },
];
scheduler.config.lightbox.sections=[
{name:“titre”, height:30, map_to:“text”, type:“textarea” , focus:true},
{name:“description”, height:130, type:“textarea”, map_to:“details” },
{name:“adresse”, height:30, type:“textarea”, map_to:“event_location” },
{name:“categorie”, height:20, type:“combo”, options: categorie, map_to:“categorie” },
{ name:“recurring”, type:“recurring”, map_to:“rec_type”, button:“recurring” },
{name:“time”, height:72, type:“calendar_time”, map_to:“auto”}
][/code]
I did not forget to include dhtmlxscheduler_editors.js; dhtmlxcombo.css, dhtmlxcommon.js, dhtmlxcombo.js (like said in documentation) 
But it doesn’t change anything 