This post shows some tips that I found to customize the dojo ComboBox Widget.

1. Change text align in drowdown menu:
Change dojoComboBoxOptions in dojo/src/widget/ComboBox.css

/* the drop down */
.dojoComboBoxOptions {
font-family: Verdana, Helvetica, Garamond, sans-serif;
/* font-size: 0.7em; */
background-color: white;
border: 1px solid #afafaf;
position: absolute;
z-index: 1000;
overflow: auto;
cursor: default;
text-align: left;
}