The panel doesn't follow VDG rules about dark theme.
The background is dark, and the font used too. 
See print-screen attached

Reproducible: Always

Steps to Reproduce:
1. Use breeze-dark theme
2. open systemmonitor
3. click right on systemmonitor -> choose detailled view
4. try to read informations


Actual Results:  
a dark background with a dark color for font.

Expected Results:  
have a better readeable panel 

I let it as normal bug level cause you can copy and paste the shown informations elsewhere.
So there's no workaround excepted changing theme each time you need systemmonitor, which is not what we can call "user friendly" ;-)