She eats vegetables and brown rice as her staple diet.
Literal
She [topic-は] vegetables [conjunctive-と] brown-rice [object-を] regular-diet [as-と] is-doing.
常食としている — 常食 (じょうしょく, 'regular/staple diet') combined with としている means 'treats as her regular food.' 玄米 (げんまい, 'brown rice') is unpolished rice, associated with health-conscious eating.