Hello Shengke,
The is no list with the available classes.
For more details, please have a look at
this discussion.
Regarding the variations - a drop-down list will be shown wherever you can use variations.
Here, have a look at
this video that I recorded for you. It show what I mean.
But the easiest solution, in my opinion, is to create your own custom CSS classes and add them wherever you want.
Here, have a look at
this video that I recorded for you. It show what I mean.