Hi Aneesh,
There is a work around so that the CSS changes do not apply on each and every charts.
Best way would be to place the chart under a div with an is say "divChart1". While you want to modify CSS for this specific chart, write your css like:
#divChart1 .sapVizChart .v-gridline {
stroke: #FFFFFF !important;
}
This would apply it for charts having "divChart1" as ID of parent element.
Hope this helps.
Best Regards,
Swaroop