Я сделал это так:
<StepLabel
classes={{
alternativeLabel: classes.alternativeLabel,
labelContainer: classes.labelContainer
}}
StepIconProps={{
classes: {
root: classes.step,
completed: classes.completed,
active: classes.active,
disabled: classes.disabled
}
}}
>
{this.state.labels[label - 1]}
</StepLabel>
А потом в классах я определил их следующим образом:
step: {
"& $completed": {
color: "lightgreen"
},
"& $active": {
color: "pink"
}
"& $disabled: {
color: "red"
}
},
alternativeLabel: {},
active: {}, //needed so that the &$active tag works
completed: {},
disabled: {},
labelContainer: {
"& $alternativeLabel": {
marginTop: 0
}
},