'Star Trek' is indeed a trademarked phrase ( a word trademark ), which is easy enough to bypass through omission. As are NCC-1701, James T Kirk, Spock etc. But to go beyond that CBS would need design trademarks on each visual element they claimed to be infringing*. Like the 1701-D bridge layout, and the captain's chair, and the shape of shuttlecraft, and the shape and decor of the corridors and... but of course it falls on the alleged infringer to prove that they haven't violate one or more that is registered.
One doesn't simply get handed an automatic blanket design trademark for every form and colour on screen. One does get copyright for what is filmed, however, to prevent it being copied into other productions.*
* For example: Lucas Arts has carefully registered design trademarks for each type of light-sabre seen in the Star Wars films.
** This being the problem with Axanar Productions. They were accused of infringing on copyright in terms of the dialogue, settings and fictional history of the episodes and films which relates specifically to "the exclusive right to develop, create, and/or produce motion pictures and television shows based on the Star Trek Copyrighted Works".