You have ways to cover the air conditioner that will keep it accessible to cool the room but also help to maintain your decor theme.
Fake Window
If you plan to leave your wall-mounted air conditioner in place, build a window frame around it to create a fake window. If you were to just hang curtains over your wall air conditioner, the curtains would look out of place. But if you use window trim to create a frame and fake windowsill, the curtain will look like it covers a window. Use a pull-string curtain rod that will allow you to easily open and close the curtains to allow access to the air conditioner.
Paint
If your wall air conditioner looks like it is stuck in the middle of a wall that has no art or any decor on it all, paint the air conditioner the same color as the wall to hide it. As long as you keep the ventilation openings and cool air release at the top clear, you can paint your air conditioner to cover it up.
Work of Art
You have two choices when using art to cover up your air conditioner. You can find an artistic piece with large openings in it to hang over your air conditioner, which will hide the air conditioner but still allow you to use it. Your other option is to put a frame around your air conditioner and paint designs on the front of the unit to make it a piece of art.
Shelving
A wall shelving unit that surrounds your air conditioner allows you to conceal your air conditioner in two ways. It allows you to put items on the shelves surrounding the air conditioner that prevent the appliance itself from standing out. The other way that shelves help is that you can put something on the shelf in front of the air conditioner to hide it.