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Doesn’t necessarily work like that, some rulesets are within a block of generic table styles, some within other sections, you won’t find a specific block of styles to easily lift out. Really your best approach if you are building as a child theme is to call in the default.css and then overwrite it with your styles in styles.css or custom.css, as you do that you can, if necessary, remove default styles as you identify them.

Using Firebug doesn’t force you to rebuild anything if you follow the general approach that is intended when using a custom.css, you still use default.css then use Firebug to identify the ruleset styling an element and copy the selectors to custom.css and style to your requirements, remove the original default rulesets if you want to or simply leave them in place.