Learn German: Warmes Öl ist gut für gesunde Haut. - Warm oil is good for healthy skin.

Because Öl (oil) is a neuter noun (das Öl) and it is the subject of the sentence (nominative case). Since there is no article like das or ein in front of it, the adjective has to do the heavy lifting and show the gender by taking the strong ending -es.

Why doesn't the adjective gut get an ending?

Adjectives only get endings when they come directly before a noun. Because gut comes after the verb ist and describes the subject from a distance, it stays in its basic, unchanged dictionary form.

Why does gesunde end in -e?

The preposition für always forces the following noun into the accusative case. Haut (skin) is a feminine noun. Because there is no article (like die or eine) in front of it, the adjective takes the strong feminine accusative ending, which is -e.

How can I tell the genders of Öl and Haut here if there are no articles?

The strong adjective endings actually give you a big clue! The -es on warmes tells you that Öl is neuter. The -e on gesunde tells you that Haut is feminine.