how to make aesthetics fit the gametype?

First of all, I'm going to assume that you mean Aesthetics.

You need to first identify what type of mood your gametype creates. For an Infection map, you might want to create a tone of fear. A Big Team Battle map would emphasize the theme of massive armies clashing. If you are doing a themed game mode, you would want to have specialized aesthetics (ex Jurassic Park map).

You also need to identify level of detail. If you are making a slayer map, maps are often pretty simple or plain aesthetically so that the visuals don't interfere with the gameplay. For something like Infection, the aesthetics are much more detailed to create a level of immersion.

After you've identified the mood, you need to find aesthetics map elements that match it. There's no right answer to this one, but looking at some maps that set a mood similar to the one you are looking to achieve can help.

Good luck with your Forging career, and hopefully I'll see your map someday. ;)

EDIT: TIL that esthetics is an alternate spelling of aesthetics (thanks Wikipedia).
 
I'm an art student.... Aesthetics makes up only 20% of my grade. So no, art is NOT equal to aesthetics.

Zabud, try to create a consistency throughout your map. This was my mistake in creating halo 4 maps. Unless it's on purpose, try to use similar architecture and textures throughout your map. Just because something looks cool on another map, it doesn't mean that it will look good on yours. Don't make one room out of concrete, and the room right next to it out of metal. You have to have a sense of continuation that will really add to the player's immersion in the map- a continuation I have yet to master.

Take a look at my Avarice map. It has a theme, yes, but WAY too many different textures. It affected gameplay and made the map look less appealing.