powerbi custom theme for stacked bar graph using json file

How we can create the custom theme for stacked bar graph using json file which include the conditional rule. i used the below code to achive the same but it's throwin error as error while importing the json file. Looks like conditional format is not supported here? it should be done manually?

"barChart": {
"*": {
"dataPoint": {
"fill": {
"solid": {
"color": {
"$expr": "d => {
if (d.value <= 20) return '#ED9AA1';
else if (d.value <= 40) return '#E77881';
else if (d.value <= 60) return '#E15662';
else if (d.value <= 80) return '#B4454E';
else return '#87343B';
}"
}
}
}
}
}
}