If nil, use the default decoration (typically the minimum available).
If t, use the maximum decoration available.
If a number, use that level of decoration (or if not available the maximum).
+The higher the number, the more decoration is done.
If a list, each element should be a cons pair of the form (MAJOR-MODE . LEVEL),
where MAJOR-MODE is a symbol or t (meaning the default). For example:
((c-mode . t) (c++-mode . 2) (t . 1))