Press Ctrl + Shift + C to open the Cheat menu then enter the following:
BoolProp testingCheatsEnabled true.

While in Create-a-Sim press Shift + N before creating your first Sim to enable skintones, clothes, and hairstles that would normally be used only by NPCs.

Press Shift + M to disable it. Bring up the Debug menu by presssing Shift + Click on a Sim while in Live mode.