。
She's wearing white clothes today.
Literal
She [topic-は] today [contrast-は] white clothes [object-を] is-wearing.
着ている is the resulting-state reading of ~ている: not 'in the process of putting on,' but 'is currently in a wearing state.' Japanese frequently uses ~ている for clothing because the result of dressing — being clothed — is what matters now. The contrastive は on 今日は flags 'today specifically,' subtly implying she usually doesn't wear white. 着る covers most upper-body and full-body garments; pants and skirts use 履く, hats use かぶる, glasses use かける — Japanese dressing verbs are body-part specific.