[Grids] Minor style update

[gvnmagni]

In order to reduce visual abundance on timeseries, I would suggest to slightly change a couple of style details of our grids.

1. Avoid dashed grid lines
   This normally happens on horizontal grid lines

2. Avoid vertical grid lines with darker color
   We use this to visually highlight relevant moments in time, but the result is that the chart look more confusing and too rich

3. Check grid lines color
   Light theme: #E9EDF3
   Dark theme: #343741

4. Bucket dashed line on hover color: EUI.colors.darkShade

The following images show the expected final result that we should get with these changes: