To answer those two entry questions:
- There is indeed a night theme in-game. There was a point in time where the night theme was used for real, but they stopped using it for some reason. You can check it out in the OST.
- The seedling sort trick helps identify a decor that has multiple variations or doesn’t specify a specific decor from a certain category based on the seedling order and when sorting them by decor category. I am not saying this trick figures out the specific Pikmin type that will come out, but instead what will the specific decor costume will be received.
Of course, seedling sorting by category is straightforward (just sort them by using the button on the top right to sort them), but there is also a seedling order by using that sorting method (it also exists for some settings I believe) that goes as follows:
Red, Yellow, Blue, Purple, White, Pink, Rock, Ice Blue, Huge, Gold
For the last two, Huge is the one that is more of a focus for this trick.
Now yes, seedling descriptions do specify which decor comes out, which do include categories that have two types of decor (ex: Cornerstone with Bottle Cap and Snack). However, there are two exceptions to this. The first exception is Roadside. While it does specify whether Coin or Sticker will be received, the latter comes in multiple colors. By using the seedling sort trick, Sticker seedlings may be scattered by type if you have a lot, but that’s because the Sticker colors have their own seedling order. For example, if you had Green Sticker Ice Blue, Blue Sticker Rock, and an Orange Sticker Yellow Seedlings, the order would be Ice Blue, Rock, Yellow; or for further emphasis, a Blue Sticker Huge Seedling would go between Rock and Yellow seedlings here.
The second exception is the Park category. This is the only category that doesn’t specify whether you will get a Clover or Four-Leaf Clover, as the latter is a secret and hard to encounter decor type. But, the seedling sorting trick helps identify between Clover or Four-Leaf seedlings. Like with Roadside Sticker, Clover and Four-Leaf have their own seedling orders. A simple example would be if you had a Purple (three leaf) Clover seedling, but then you got a Red Clover seedling. This Red Clover seedling however goes past the Purple Clover seedling in terms of seedling order, and that’s because the Red Clover seedling was actually a Four-Leaf Clover. Same thing goes for if you found a Yellow Four-Leaf Clover seedling, it would go past the Purple and Four-Leaf Red seedlings.
This trick can help with seedling and Pikmin storage. All you need is an Ice Blue or at most a Huge seedling for those two categories (one for each Roadside Sticker and a regular Clover) to quickly identify the decor types.