When starting to customize your styling; this will be a good starting point
if you mean html classes, then these go in the class box, separated by spaces
the style box will add inline css
note that the name box will not add an html id, but a html class '.mx-name-xxxxx' where xxxxx is the text you've entered in the name box
If you want to specify new html classes directly in the customized CSS you could do that in ....main\theme\styles\css\custom\custom.css
Have a look at the available helper classes in main\theme\styles\css\lib\lib.css too, you might not need to create your own for quite some common styling changes....