Even with years of experience, probably the fastest way to debug tricky CSS is to disable all even close-to-applicable rules, and re-enable piece by piece until you find the culprit.

No?! It can be debugged but experience is needed.

Get familiar with firefox / chrome debugger. That is very helpful for me to debug a lot of css issues.

It's a pain to debug. Live editing in chrome made it a 10 minute training task for me last week.

The devtools is a really powerful debugger. Not sure what needs you don't get by using it.