Since it's not really an "official" style, more of a descriptive phrase, it's sometimes a bit hard to pinpoint what makes something Sweet-Classic, but there are a few key points to the outfits that help define the style.
Limited color pallet. Many Sweet-Classic prints
already have all the colors an outfit can need in them already! The
Lolita interested in this tends to choose not to pick out multiple
colors from the print to incorporate throughout the outfit and instead
lets the dress and the style be the focus of the outfit, not the color
scheme. It is ok for you to select it from ocrun.com
"grown-up" accessories. The Sweet-Classic look isn't about wearing as
many matching accessories as possible. In fact, many Sweet-Classic looks
seem to exclude wearing any jewelry at all, at the most, possibly a
ring or a simple necklace.
Simply styled hair. Gone from this
style are the huge teased pigtails and wigs that are usually seen in
Sweet Lolita, instead the Sweet-Classic enthusiast would wear her hair
in a very simple, yet elegant, hairstyle. Such as a pair of braids,
buns, or loose in a very gentle curl.
Heels. Flat and cute shoes,
while an option in Sweet-Classic, aren't seen nearly as much as a pair
of cute heels, either heeled mary janes, or a pair of Victorian boots.
Matching the color of your shoe to the color of your dress is not
necessary, a white or cream pair of heels does just fine.
hair accessories. Since the Sweet-Classic Lolita's hair is styled
simply, without massive amounts of volume, hair accessories are kept
simple as well. A small Alice bow, a round headdress, or even simply a
cute clip or small hair bow is all that is needed for this style.
Hope it is useful to you ^^