Using the Web Inspector in ProfitTrain (or any WebKit-based application)
This is a little technical, but for those extreme CSS crafters potentially useful.
ProfitTrain like many apps on Mac OS X utilizes the WebKit framework to render web pages. This is the same engine that powers Safari.
WebKit has a thing called the Web Inspector:

The Web Inspector is useful to debug CSS code and figure out which elements are getting which styles.
If you are a ProfitTrain invoice template designer and would like access to the Web Inspector while editing a template design follow these steps:
Make sure ProfitTrain is not running, and then via terminal execute the command:
defaults write com.clickablebliss.ProfitTrain WebKitDeveloperExtras -bool trueLaunch ProfitTrain and highlight an invoice. Traditionally you’ll be viewing the PDF representation in the view pane. Use the menu bar to switch this view to use the WebView.

Right click inside of the WebView and choose the Inspect Element menu. This will open the Web Inspector inside of the WebView. To detach the inspector click on the little button in the bottom left.


And now you can use the inspector to debug the CSS as well as other things.
Thanks to forum user fabiopigi for bringing up this need and getting me to figure out how to turn it on.