[FR] Allow custom CSS for xgrid and ygrid

[lightmaster]

I love this temperature graph, however when using a dark OctoPrint theme, the x and y grid lines are basically the same color as the background. I tried to write some custom CSS using UI Customizer, however i can't get .x .xgrid to accept any new stroke value. Admittedly I am not very well-versed in CSS so I could be going about this entirely wrong. Custom CSS I tried to use just to get the color the change is:

.x .xgrid {
  stroke: #FFFFFF;
}