dark-graph.css

[id^='highcharts'] rect.highcharts-background,
[id^='highcharts'] .highcharts-tooltip > path:last-of-type {
  fill: rgba(0, 0, 0, 0.5) !important;
}
[id^='highcharts'] rect.highcharts-selection-marker {
  fill: rgba(255, 255, 255, 0.5);
}
[id^='highcharts'] .highcharts-xaxis > path,
[id^='highcharts'] .highcharts-yaxis-grid > path {
  stroke: #404040;
}
[id^='highcharts'] .highcharts-xaxis > path:last-of-type {
  stroke: rgba(204, 214, 235, 0.5);
}
[id^='highcharts'] text[style*='weight:bold'] {
  fill: #a0a0a0 !important;
}
[id^='highcharts'] text[style*='weight:bold'][style$='fill:#000000;'] {
  fill: #707070 !important;
  text-shadow: 0 0 2px #333,0 0 5px #333;
}
[id^='highcharts'] text {
  fill: #707070 !important;
}
[id^='highcharts'] text:hover {
  fill: #a0a0a0 !important;
  text-shadow: 0 0 2px #333,0 0 5px #333;
}