There is only 1 button-icon-right per widget-accordion-group right? You might have to do the odd / even distinction at a higher level and once you've got the selected one go in through nesting and set the specific button to whichever is desired.
so as follows:
.widget-accordion-group:nth-of-type(odd) {
.widget-accordion-group-header-button-icon-right {
background-color: blue !important;
}
}