The theme was used on a few aspects, though the only really overt ones are the limited timer to clear a stage and limited play area where mostly everything fits into the camera. In a less obvious case, the player can actually only have up to 12 shots onscreen at once (equivalent to firing 4 times), encouraging pointblanking enemies so you never hit that limit.
Subtler uses of the theme comes from the scoring system, all of them being different flavors of time limits (a timer for the long-term gap kill main multiplier, a timer for the short-term dash kill multiplier, and timers to collect the gold spawned by dash kills before they despawn).