In a temperate climate where deciduous trees drop their leaves for the winter such leaves are nearly the perfect compostable material.  Trees drop them to mulch the ground for one season and then decompose to return useful nutrients to the soil where those same trees can use them again.

A reasonable generalization for the carbon to nitrogen ratio of recently fallen tree leaves is 30 to 1.  The ideal carbon to nitrogen ratio for a home compost pile is 30 to 1.

The nitrogen level in tree leaves *does* begin to drop soon after they fall so the more quickly you can get the leaves into the compost pile (where the nitrogen can be captured by your composting bacteria) the better.

Thus, recently fallen tree leaves are probably a better composting material than the average household food waste.  Adding tree leaves to your household food waste compost pile will buffer the carbon to nitrogen ratio and let you successfully compost material that is further from the ideal ratio.

One issue to watch out for, though, is that mulch goal.  A thick, matted layer of leaves will create a layer in your compost pile that is difficult for water and air to penetrate.  Starving your pile of air will turn it anaerobic and nasty.  The stink of an anaerobic compost pile is the smell of volatile organic compounds escaping - likely including methane.  To avoid this, mix leaves thoroughly into the pile or shred them before adding.

If your compost pile ever goes anaerobic (because of tree leaves or any other reason), promptly turn and aerate the whole thing and correct whatever management practice caused the problem.