connectDaily uses font-based CSS icons. The CSS Classes of the icons are in the cdaily-5.0.0/WEB-INF/classes/css-icons.properties file.
If you want to change them, copy the calendar-icons.properties file to custom-icons.properties and then add the line
IconsFile=custom-icons
to your configuration.properties file. If you're running the hosted version, copy and paste the line into the Import License Data screen.
By creating a custom icons file you do not have to worry about a future upgrade overwriting the icons or the icon list file.
Edit the custom-icons.properties file, replacing the names of the default icons with the names of the icons you want to use.
Upload or copy the custom-icons.properties file to the WEB-INF/classes directory for your connectDaily installation.