You can take a screenshot (or grab it with the snipping tool) and load it into Paint.
In Paint there's a tool called "Color picker" (the icon looks like a pipette).
You pick the color - and then you can see the color code in the pop up when you click the "Edit colors" button.
I would have found it for you, but I'm not sure which theme you're talking about: Also by default, I think Windows 10 is set up to calculate the color from your desktop background image.