When adding a stylesheet containing
.logo-highlight {
-fx-background-color: radial-gradient(radius 100%, center 50% 50%, red, darkgray, black);
}

I get the error "Cannot add stylesheet. null"

To make stylesheets easier to debug, I suggest you provide the line number and character index of the error along with a message stating what kind of problem was encountered. Something like "Unable to read parameter <parameter name> because <something> is not <something else> at line 20 character 33".