Not working on recurring events

What is the issue? Please use screenshots of your Category Colors settings and activate the legend.

CSS works by having later declared code override previously declared code. The CSS selectors created by Category Colors are identical and so if you have more than one category assigned the last declared category will take precedence. Categories are written out in alphabetical order.

Your recurring events have 2 categories assigned to each one and the last category has the default grey border/background.

You can hide these categories in the settings and no CSS will be created, or you can have one category as a border and one as a background.

Displaying the legend will make this more clear.