When having a lot of placemarks groups showing on the same map ("Manage Placemarks" and "Synchronize with placemark list" are both active) it would be really nice to have som sort of indication of whether a group is active (visible) or not.
Go to map details.
Open tab Groups
Field "Group CSS"
There are 2 predefined styles
Set Advanced and you'll see difference
You can also set External, and in your template add styles for that case.
In style classes will be "-external" suffix.
You can copy styles (with rename it to -external) as you need from my CSS file into your template and change what you need in case "External" style
Just set different values and check how I switch class names depend on two fields "Group CSS" and "Group Control"