It was always 30 for me. But I never had more than 30dp (level wise) when I completed the game.
It could be that you need 30dp to change until your dp (by level) is greater than 30. After that, you need the amount of dp you have.
dp is obtained and lost by card breaks. Whenever you break an enemy card you gain the difference in breaks. ie If you play a 9 and the enemy had a 3, you gain 6 dp.
If you play a 5 and the enemy plays a 8, you lose 3 dp from your counter.
When your dp counter reaches 30, you turn dark. While in dark mode, i don't think youc an gain more dp by breaking enemy cards, but being card broken takes away your dp. when your dp reaches 0, you turn back to normal mode.