After you click the link, you must select Pearl 8100, then use the first download link.
The backup during the upgrade will preserve the Theme. Just make sure you delete the vendor.xml file or you won't be able to upgrade.
Man those '91s stock were harsh for me back in the day. I owned a 1989 Nissan 300ZX Turbo that was crazy fast at the time with a 205HP, 3.0 liter, 12-valve SOHC V-6. When Toyota released the updated Supra (ie. yours) I believed it had either a 3.2 or 3.4 liter 24-valve DOHC V-6 and an Intercooled Turbo pushing 232HP. Whew, then in 1991, Nissan gave me my next gift and I bet you know what that was......now I drive a 3/4 ton crew cab pickup that weights 7,000 pounds.
Nice ride man!!!!