Hi @fv and welcome to the Lithium Community,
You are right in that overwriting CSS definitions is not the ideal way to do. Luckily the SASS file structure allow you to overwrite in a way that you are not bloating the output definitions, but only the latest definition from your skin will actually make it through.
Theoretically you could start from scratch, but it would be cumbersome and potentially prolonging your deployment times as it's quite tricky to work out from the unstyled structure how page elements might be intended to be positioned.
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