horizontal category list display

No version of the My Calendar stylesheets sets the category list to be block level, although the ‘refresh’ style sets the category key list to have a columnar display, which can look similar to a block level display. Otherwise, the lists are simply inheriting whatever your theme has established for them, or the default display value of ‘list-item’.

If you want them inline, you should be able to do that using .mc-main .category-key li { display: inline; }. If you’re using the ‘refresh’ style, you may also need to remove the column-count settings.