Learn Finnish: Tämä metsä on niin rehevä kuin pimeäkin.

This is a set phrase used to link two things, often for emphasis. You put niin before the first item (rehevä), kuin before the second (pimeä), and attach the enclitic particle -kin (meaning "also") to the end of that second item. It literally translates to something like "so lush as it is dark too".

Could I just say "sekä rehevä että pimeä" instead?

Yes! sekä ... että is the standard, everyday way to say "both ... and". The niin ... kuin ... -kin structure used here is just a slightly more formal or literary alternative that adds a nice stylistic variety to your Finnish.

If the forest was in a different case, where would the -kin clitic go on the second adjective?

The -kin clitic is always attached to the very end of the word, after any case endings. For example, if you said "in this lush and dark forest" (using the inessive case -ssä), it would be niin rehevässä kuin pimeässäkin.

Does rehevä only refer to plants and forests?

In nature contexts like this sentence, rehevä means "lush" or "overgrown". However, if you use it to describe a person, it means "curvy", "buxom", or "voluptuous". It is a good distinction to remember so you do not use it accidentally in the wrong context!