John Hull. Options, Futures, and Other Derivatives (2017 10 edn). p 408.
Theta is usually negative for an option.7 This is because, as time passes with all else remaining the same, the option tends to become less valuable.
7 An exception to this could be an in-the-money European put option on a non-dividend-paying stock or an in-the-money European call option on a currency with a very high interest rate.
u/BANG_BANG_SHRIMP alleges
Theta works in your favor if you are selling debit spreads.