@wu_weidong one of the options would be to turn on "Dark Theme" for your dashboard. This will automatically invert the Tooltip background Color and font color.
Second option could be to use Simple XML CSS Override to style the Tooltip as per your needs. If the chart ID is chartWithTooltipCSSOverride then following html panel with CSS override can be added to test. Once finalized, you should move CSS override to CSS file and reference the same in your dashboard/s for reusability.
<html depends="$alwaysHideCSSOverridePanel$">
<style>
#chartWithTooltipCSSOverride g.highcharts-tooltip path {
fill: white !important;
}
#chartWithTooltipCSSOverride table.highcharts-tooltip tbody tr td:last-child{
color: black !important;
}
</style>
</html>
<chart id="chartWithTooltipCSSOverride">Please try out and confirm!
