At the conference in Las Vegas, Bruce recommended not changing the all.css file because he will be making some changes to it in the future.
He recommended creating a new style file (custom.css) and copying in the relevant sections from all.css and then changing their name.
For example, the table.BrowseTable might become table.CustomBrowseTable (or whatever you wish to call it) in the custom.css file. You then include the new css file in the Styles section under Settings in the NetTalk or NetSimple Object.