When you have a functional body, you can put a skin on and probably add some make-up.

Look and feel is an important part of UX. It creates a mood for each screen. Well thought look and feel shows up the dedication of the company that created the product.

It has to be consistent in terms of style unless there is a special intention (that could happen in a game) and have enough contrast so users can see elements they should see.

As soon as you have first page with look and feel applied, show it to end users. Aesthetics could be subjective, but make sure the purpose of the basic user interface elements stays clear.