It’s not possible to customize the thickness & style of one particular gridline. Thickness & styling applies for all the gridlines. However, you can add stripline at specific value with custom color & thickness as per your requirements.
Gridlines are drawn at every interval of the axis. Color & thickness of all the grids can be changed using gridColor & gridThickness properties respectively. As addressed earlier, you can use stripline at a particular value to show different thickness & color of a grid. Please take a look at this JSFiddle for an example.