SonarCloud reports the potential issue with MetalHighContrastTheme.getControlHighlight:
Refactor this getter so that it actually refers to the field "controlHighlight".

   public ColorUIResource getControlHighlight() {
        // This was super.getSecondary3();
        return secondary2;
    }

Indeed, there is a field:

    private static final ColorUIResource controlHighlight = new
                              ColorUIResource(102, 102, 102);

...that is not used. Either we should use it, or remove the field declaration?