Theft of the Winter Star adds a new dungeon area before the Feast of the Winter Star festival, and a new late-game farming equipment if you beat it.

Install

Install the latest version of SMAPI, Json Assets, and SpaceCore.
Unzip the mod folder into Stardew Valley/Mods.
Run the game using SMAPI.

Use
Overview
Someone stole the decorations for the Feast of the Winter Star! When you leave the farmhouse in winter (anytime before winter 25), Lewis will tell you about it and start you on the path for the quest.

Beating the quest line will give you a stardrop, a friendship bonus with the entire town, and the Tempus Globe recipe.

Tempus Globe
Beating the dungeon gives you the recipe for the Tempus Globe, a mystical item which can be crafted and placed on your farm. Crops in a 5x5 area around Tempus Globes will grow year-long and be auto-watered.

Crafting a Tempus Globe requires an iridum sprinkler, 25 Solar Essence, and 25 Void Essence.

Solution

Exit the farmhouse in winter (before winter 25), and a cutscene with Lewis will play.
Walk into the tunnel at the bus stop.
Interact with the strange hole at the end of the tunnel to enter the new dungeon.
Right-click the bag in the second room to get the first key. (Do it again for the first frosty stardrop piece.) Use the key on the first door in the previous room to proceed.
In the room with three pedestals, place (in order) a Frozen Tear, Solar Essence, and Void Essence. You'll receive another frosty stardrop piece, a key, and a Festive Scepter. You can
also bomb the wall here for another frosty stardrop piece.
In the room with the boss door, head left and solve the maze for the boss key (half A). The door in the top-left leads to a target you need to hit with a projectile for a frosty stardrop piece.
In the room with the push block puzzle, you need to push a white block onto the blue tile for the boss key (half B). You can also bomb the wall here for another frosty stardrop piece.
At this point you can craft the frosty stardrop pieces together for a Frosty Stardrop.
Put the boss key halves onto the pedestals to open the door and fight the boss.
Beat the boss to get half a heart with the entire town, and a recipe for the Tempus Globe.

Compatibility
Compatible with Stardew Valley 1.5.5+ on Linux/macOS/Windows, single-player, but not multiplayer.

See also