From the author's README on Thunderstore

Classic Sonic T-Shirt

Adds a Blue Classic Sonic T-Shirt to your shirts apparel.

Install

Install from the ReSkate launcher's MODS MANAGER section, or drop the folder into the Mods\folder beside Skate.exe and the 'ReSkateLauncher.exe'

Install 67ClassicSonic

You need ReSkate and your own copy of skate. on Steam.

Open ReSkateLauncher.exe and go to MODS.
Search for 67ClassicSonic and click install. The launcher also installs anything it depends on.
Press PLAY. In game, the MODS tab of the ReSkate menu (Insert) turns it on and off.

Or install it by hand: download the .zip and drop it on the launcher window, or unzip it into its own folder in Mods\ beside Skate.exe.

Maps show up in the ReSkate menu's level list. To join a server running a custom map, you need the same map installed. If the game won't load after installing, see known issues.

skatemods doesn't host or review this mod. The download comes straight from Thunderstore. Only install mods you trust, because scripts can run code.