A few quotes, at first on :
Option spreads quenching theta
A bullish call spread consists in buying an ‘in the money’ call option and selling simultaneously an ‘out the money’ call option with the same expiry date. You would preferably choose a quarterly option (or a yearly leap) which has plenty of time left before expiry. Your investment equals the price difference between the bought call and the written call. It also is the total ‘value at risk’.
For the two Greeks of that spread we obtain: (δ, q) = (δ1 - δ2, q1 - q2). The expensive ITM call has an intrinsice value equal to (share price – strike). Its time value is (option price – (share price – strike)). The out-the-money written call is all time value. You notice that the combined q is the difference of the two q’s of the components long (bought) and short (written). Depending on how you choose the strike levels relative to the current stock price, the combined q can be around 0 or even slightly negative: an option spread has a weak time dependency. The time value loss of the ITM call is compensated by that of the OTM written call.