What is the opposite of subtle?
The opposite of subtle could be obvious.
Depending on the context, it could also be overt, harsh, forthright, or coarse.
If a custom is taken to mean a habit or tradition, it's naturally occuring opposite would be the lack of a habit or tradition. A slightly philisophical view point but it seems that the opposite of something is not the presence of something else completely unrelated, as wet is to dry for example. The opposite of wet is the absence of wet which may also happen to be the same as dry but when you…
Try to be subtle when you ask about how much the motel room costs. The subtle change in her tone probably meant that she knew we were listening. We made some subtle alterations to the blueprints before construction. "The universe is wondrous, with treasures to satiate desires both subtle and gross." (Q to Picard, Star Trek the Next Generation)