Hi! You'll need to add your CSS to the config via the global_css setting (as a single string) – also see the example in my comment here.
If you want to have equal columns, you shouldn't have to set any style on the children, just define the columns of the parent block. If you do want to have custom columns for the different options, the easiest way is to target them with :nth-child in the global CSS (so you have all styling in one place). If you want to define styles on the individual options, you can do that, too – but it needs to be a dictionary with camel-cased options (as shown in the data format example here).