I styled a combobox through css.
When running an app with these combobox, after "opening" the combobox, its border disappears, never to return again.

You can find the sample app here:
https://github.com/JFXtras/jfxtras-styles/tree/master/src/jmetro8

The relevant files are:
JMetro ComboBox.fxml
JMetroLightTheme.css (In this file the code that is under the combobox section)
Main.java

Run the app with:
static final String RESOURCE = COMBOBOX_RESOURCE;
static final String STYLE_SHEET = LIGHT_STYLE_SHEET;