I have a gradle project in eclipse. Here is the structure of my project
I have css resource styleclass.css in scr/main/resources/css
You should put your css in the same package as the class which uses it, then use Class.getResource to access it, with the relative Path.
src/main/java:
pk.training.basit
ComboBoxStyling.java
src/main/resources:
pk.training.basit
ComboBoxStyling.css
Then in the source:
scene.getStylesheets().add(
ComboBoxStyling.class.getResource("ComboBoxStyling.css").toExternalForm());
Personnally I think it's even a better practice to put the css and the class in the same directory, then configure gradle so It can take the resources in the src/main directory. It permits to have an architecture analog to other GUI products (Angular) which places all the resources of a screen including the source in the same directory, the FXML file for example if you had one.