I'm afraid I haven't worked with Cartpauj PM since I was putting together a demo for a group some months back - but I'm sure I can find my way back to where I changed things, since I still have all the files...
Looking at them, it seems I did NOT change this through the child-theme's regular style.css file - but rather through Cartpauj PM's own style.css file, which is found in the /wp-content/plugins/cartpauj-pm/style folder.
Please look at this file and see changing the values in it gives you the results you want.
Normally, I would copy any changes like this into the child-theme's main CSS file, but on occasion I've found this won't work and you need to change the plug-in's CSS file instead. This means your changes may not survive an upgrade, but perhaps marking them as !important in the child-theme's CSS would get around this...
Let me know, and I'll be glad to offer any help I can.