Effects that modify a creature’s power and/or toughness, such as the effect of Titanic Growth, will apply to the creatures no matter when they started to take effect. The same is true for any counters that change their power and/or toughness and effects that switch power and toughness.

Polymorphist’s Jest doesn’t counter abilities that have already triggered or been activated. In particular, there is no way to cast this spell to stop a creature’s ability that says “At the beginning of your upkeep,” “When this creature enters the battlefield,” or similar from triggering.

Polymorphist’s Jest overwrites all previous effects that set the creatures’ base power and toughness to specific values. Any power- or toughness-setting effects that start to apply after Polymorphist’s Jest resolves will overwrite this effect.

If one of the Theros block Gods is affected by Polymorphist’s Jest, it will be a legendary 1/1 blue Frog enchantment creature with no abilities. If it stops being a creature, perhaps because your devotion to its color(s) decreased, it will be a legendary blue enchantment with no abilities. The way continuous effects work, the God’s type-changing ability is applied before the effect that removes that ability is applied.

Creatures that enter the battlefield or come under the target player’s control after Polymorphist’s Jest resolves won’t be affected.

The creatures will lose all other colors and creature types, but they will retain any other card types (such as artifact) or supertypes (such as legendary) they may have.

If one of the affected creatures gains an ability after Polymorphist’s Jest resolves, it will keep that ability.