How to style a historical barchart in nvd3 - css

Question

I am using nvd3 to create a historical barchart and it's working fine. There are two things I would like to do with the bars.
style the bars with rounded edges instead of square.
colour the bars based on their values
For the second case, I tried a function as follows and it doesn't work.
function(d,i) {
return d.y > 50? "red":"blue";
}
Updated:
This is the data that I am using. I am just trying to colour the bars based on the value. So if the data value is more than 50, it should colour the bar red. As for the other question, I just want to style them with rounded edges.
data = [{
"values" : [
[ 1136005200000 , 17.0] , [ 1138683600000 , 12.0] , [ 1141102800000 , 12.0] , [ 1143781200000 , 14] ,
[ 1146369600000 , 20] , [ 1149048000000 , 21] , [ 1151640000000 , 17] , [ 1154318400000 , 34] , [ 1156996800000 , 10] ,
[ 1159588800000 , 8.0] , [ 1162270800000 , 38.0] , [ 1164862800000 , 38.0] , [ 1167541200000 , 35.0] ,
[ 1170219600000 , 55.0] , [ 1172638800000 , 35.0] , [ 1175313600000 , 26.0] , [ 1177905600000 , 26.0] ,
[ 1180584000000 , 26.0] , [ 1183176000000 , 25.0] , [ 1185854400000 , 25.0] , [ 1188532800000 , 25.0] ,
[ 1191124800000 , 29.0] , [ 1193803200000 , 29.0] , [ 1196398800000 , 29.0] , [ 1199077200000 , 52.0] ,
[ 1201755600000 , 22.0] , [ 1204261200000 , 22.0] , [ 1206936000000 , 22.0] , [ 1209528000000 , 22.0]
}];

Answer 1

For example, this code draws a different shade of blue depending on the x-value, and grey if the y-value is zero:
d3.selectAll("rect.nv-bar").style("fill", function(d,i){
var colorCode = "#1f77b4";
var date = new Date(d.x * 1000);
var hours = date.getUTCHours();
if(hours < 6 || hours >= 18){
colorCode = "#355880";
}
if(d.y === 0){
colorCode = "grey";
}
return colorCode;
});