GOT IT!
Looking at the QT-table showed me the way ;-)
Find the json-file
...\Calibre\Calibre Settings\plugins\Job Spy.json
make sure Calibre is closed, and just replace the line
"GUI_TOOLS_LIBRARY_VIEW_ALTERNATING_COLOR": "midnightblue"
with, for instance
"GUI_TOOLS_LIBRARY_VIEW_ALTERNATING_COLOR": "#101010"
Note that you need to redo this after every manual-according-to-the-book settings modification in the Job Spy plugin...
I'm very happy!