I prepared a jfiddle with only country borders in order to help you and all styles resetted:
var styles = [
{
"stylers": [
{ "visibility": "off" }
]
}
];
I'd like to change colour of both country borders and coast lines! Thanks!
Edit – august 2021
Google Maps API V3 currently accepts administrative.country
directive for styling country borders.
{
"featureType": "administrative.country",
"elementType": "geometry.stroke",
"stylers": [
{
"color": "#ff0000"
}
]
}
Directive for coastlines is unfortunately not supported.
Original answer
There is an issue with the API:
Issue 7165: Bug: Cannot hide country borders anymore (since 9/24/14)
administrative.country.geometry
elements seem to accept no styling at all.
related question: Google Maps: Hide country borders