First, locate the most inner elements (typically DIVs) with these classes:
.custom-widget
.custom-widget-content
Change the background property to a color of your choice and observe the result. If the background doesn't change, move up a parent DIV/element and continue experimenting until you achieve the desired outcome. For instance, by adjusting only 2 rules, the background of the sample1.html demo (from the ptTimeSelect download) was modified.
#ptTimeSelectCntr .custom-widget{
padding: .2em;
background:#900;
}
#ptTimeSelectCntr .custom-widget-content {
margin-top: .1em;
margin-bottom: .1em;
padding: .2em;
background:#900;
}
I hope this explanation is useful for you.