I think that at least default theme shall have valid CSS format.
Just take a look, how many errors there: http://jigsaw.w3.org/css-validator/

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Steven’s picture

Steven commented
Status: Active » Closed (won't fix)

http://jigsaw.w3.org/css-validator/validator?uri=http%3A%2F%2Facko.net%2...

It is valid CSS3. I only see 2 errors when validating against CSS2.

Instead of staring yourself blind on validators, go learn the intention and principles behind CSS:
http://www.w3.org/TR/CSS2/intro.html#q6
and
http://www.w3.org/TR/CSS2/syndata.html#parsing-errors