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Some recommend copying the default CSS over to your stylesheet.

Benefits of this approach is one less data request and full control over your styles (good when you’re designing a completely different look from bp-default).

Negative is if your child theme completely relies on the bp-default theme with little design changes, you might as well use the @import- declaration so you can easily override selected style declarations on a case-by-case basis. Some theme designers frown upon using !important, but at the end of the day, it’s your call.