。
The hills and fields are dressed in spring colors.
Literal
Hill-field [topic-は] spring [gen-の] attire [copula-だ].
野山 ('hills and fields,' 'the countryside') is a fixed compound. 装い ('attire, dress, appearance') is metaphorical here — the countryside 'wears spring clothes,' i.e., is clothed in its spring look. Poetic/literary feel, the sort of phrase you'd find in a haiku prose introduction.