I think this is interesting for everybody:
In File dhtmlxscheduler.js serach next function: scheduler._reset_scale
Then search this lines:
var cls = "dhx_scale_holder";
if (d.valueOf() == today.valueOf()) cls = "dhx_scale_holder_now";
And add below this one:
if (d.valueOf() < today.valueOf()) cls = "dhx_scale_holder_now past";Then add the next css class and set style you want for past days:
.dhx_scale_holder_now.past{
border-right:1px solid #586A7E;
opacity:0.4;
}