Hi Kieron,

I am able to replicate this. It looks like these are using the CSS property "--now-datavis_color--qualitative-X" [X is a number] which then reference other CSS properties within the theme record. You can view which color matches on the UX Theme Property from the color pallet. In my screenshots, I am using Default UI14 and I'm only showing the first color:

From here, you can look for this property on the dark theme under Now Experience Framework > Themes > Themes in the Polaris record:

Dark theme:

Base polaris "colors"

So in dark theme the first blue is following "--nowdatavis_color--blue-8" (128,218,235) while the default is using "--nowdatavis_color--blue-5" (10,184,217)

In a custom theme, you would be able to edit these values to be the same across reports.

-Matt