I made a backup of my style before (I thought) .. but it just made a backup of Master Style and seems there's no "Customized in this Style" include.
When importing the calendar style, it should not delete existing CSS if possible, it should only add the calendar CSS.
If it's not possible to keep existing CSS, then there should be a special info about in the install instructions.
Lucky me I had a 2 months old backup copy of my style, although its outdated, it saved my day.