CRTs consume so much power that the beam current is negligible. The comparison is with backlit LCD screens, which have to have the backlight on regardless of how many black pixels you have.
Actually traditional tft displays need to apply a current to turn a pixel black, so if you don't use adaptive brightness that would dim the backlight if the display is mostly black, you actually consume more power with a black theme. I have a low power Pentium notebook with 15" led backlit screen and the difference between a terminal with black or white background is significant.
Battery at 74%. Sitting at black screen for about a minute, the highest estimate I got was 4:50, lowest 4:05. It was mostly showing numbers close to either of these, almost never something in between.
Switching to white I got 5:40 and 4:50 showing pretty much the same behavior.
It's an HP pavilion 15-p005ng with a Pentium N3530. Terrible machine btw. Linux freezes in irregular intervals, almost immediately with OpenGL stuff...
I'm pretty sure that's how it works. The drivers report total battery capacity, current battery capacity, and current power draw, so the meters just do the math.