Learn Croatian: Djevojka tamnog tena je vrlo lijepa. - The girl of dark complexion is very beautiful.

This is the Genitive of quality. It is used to describe an inherent characteristic or attribute of someone or something. Instead of saying the girl 'has' a dark complexion, Croatian places the descriptive adjective and noun into the genitive case directly after the noun they describe, literally meaning 'a girl of dark complexion'.

Could I use the instrumental case and say djevojka s tamnim tenom instead?

Yes, using s plus the instrumental case is also grammatically correct and very common in everyday speech. However, the genitive of quality (tamnog tena) is often preferred in writing or slightly more formal contexts to describe permanent physical traits, as it sounds more elegant.

Since tena ends in '-a', is it a feminine noun?

No, the base word is ten (complexion or skin tone), which is a masculine noun. Because it is masculine, it takes the -a ending in the genitive singular, making it temporarily look like a regular feminine noun in the nominative.

Why is lijepa in the nominative case if it immediately follows a genitive phrase?

The adjective lijepa (beautiful) describes the main subject of the sentence, djevojka (the girl), which is in the nominative case. The genitive phrase tamnog tena is just an attribute attached to the subject, so it does not affect the case of the adjective that completes the verb je.