Pdr1
The answer is “ partly “ the fuel tanks are kept at slightly above ambient pressure, a NACA duct is installed under each wing ( outboard ) to apply air pressure into the vent /surge system.
This has a number of functions, it prevents cavitation in the boost pumps and in the event of a double boost pump failure helps the main tanks to gravity feed.
Some how with temperatures in the range of -40c I can’t see anything boiling off, in fact the most likely fuel issue at altitude is waxing of the fuel if the temperature goes below about -45c, this can cause blockage of the fuel filters.
The later B737NG Aircraft also have a system to increase the nitrogen levels in the fuel tanks, this is intended to reduce the ( very unlikely ) risk of explosion. The risk of this is highest on a hot day when the centre tank is almost empty.