If I mouseover any charts, the default background color of the "popup" is black and the text is white/other colors. However, that makes it quite difficult to read what the text is sometimes. Is there a way to change the background color to white and the white text to black? Similar to the "popup" when I mouseover a choropleth map?
@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.