in the mainstream (of something)
Widely accepted and/or normalized. Lots of ideas currently in the mainstream weren't at one time. I mean, not that long ago, women and minorities in this country weren't allowed to vote. That hairstyle definitely isn't in the mainstream of fashion, yikes.
*in the mainstream (of something)
following the current trends or styles that are popular or are considered normal. (*Typically: be ~; get [into] ~.) Bob is too old-fashioned to be in the mainstream of modern living.