[Solved]-d3 click handler not working on leaflet map layer-d3.js

The problem is that the style attributes are not valid for the svg element.
Try...

feature.on("mouseover", mouse_over);
function mouse_over(d) {
    d3.select(this) 
        .transition()
        .duration(500)
        .style("fill", "lightBlue")
        .style("stroke", "white");
      }

Or...

feature.on("mouseover", mouse_over);
function mouse_over(d) {
    d3.select(this) 
        .transition()
        .duration(500)
        .style({fill: "lightBlue", stroke: "white"});
      }  

Or...

feature.on("mouseover", mouse_over);
function mouse_over(d) {
    d3.select(this) 
        .transition()
        .duration(500)
        .attr({fill: "lightBlue", stroke: "white"});
      }