[O] Deadline Warning Days Faces

[SW]

Apologies if this is a basic question. Org-mode is so huge that it's sometimes
difficult to know where to look. I have several DEADLINE entries set with "-99d"
advanced warning to ensure they appear on my agenda. Entries due in, for
example, 30 or less days are in a different colour to those due in 50 or more
days. What variables control this behaviour? Where could I have looked/searched
for this? Thanks.