Can adjectives not only modify a noun but also a noun phrase? In the same way, can adverbs not only modify an adjective but also an adjective phrase? Why don’t any grammar books say modifying a phrase?
According to The Cambridge Grammar of The English Language (Huddleston & Pullum, 2002), adjectives can function as an external modifier to a noun phase, in which case, it does not modify a nominal (noun phrase minus determinative), but a noun phrase. For example, "such" modifies a noun phrase in "such a nuisance". Note that this rule is only applied to a small subset of adjectives such as "such" and adjectives that are modified by "too" and "so".
Adverbs, on the other hand, can freely modify an adjective phrase. There's no constraints whatsoever. For example, "occasionally" modifies an adjective phrase "very offensive" in "his occasionally very offensive behavior".