Here is the original item:
You might think (A)
that a floor-length dress is
conservative (B)
until you see the one that Christina
wears. (C)
A psychedelic swirl of pinks and purples that attracts everyone's
attention.
You wanted to do this:
You might think,
that a floor-length dress is conservative until you see the one that
Christina wears. A psychedelic swirl of pinks and purples that attracts
everyone's attention.
Adding a comma after think is an error. That a floor-length dress is conservative is a subordinate clause. When you connect a subordinate clause to the end of a main clause, you generally do not use any punctuation.
You might want to review the rules.