Actually, on the Mac you need to replace the style.css file inside the program itself (/Applications/BeerSmith2.app/Contents/Resources/style.css) because every time you choose a different report in BeerSmith app, the file in BeerSmith2/Reports/style.css gets replaced by the version from inside the app.
It would be better if the app would copy the style.css file from the app only if it doesn't exist in BeerSmith2/Reports/. This would allow us to keep our modified style.css file in BeerSmith2/Reports directory.
Failing that, we need to customize our report htm files by coding the location of our style.css file from:
<link rel="stylesheet" href="./style.css" type="text/css" /> to <link rel="stylesheet" href="./Custom/style.css" type="text/css" />