JDK: 7b78+
Platform: All

Method SynthTextPaneUI.installUI(JComponent c) says:

Installs the UI for a component. This does the following things:
    ...
    2. Installs the default caret and highlighter into the associated component.
    ...

It doesn't happen. Please see the following example:
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package synth;

import javax.swing.*;
import javax.swing.event.ChangeListener;
import javax.swing.plaf.synth.*;
import javax.swing.text.*;
import java.awt.*;

import static java.lang.System.*;

public class Defaults {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override public void run() {
                doTheTest();
            }
        });
    }

    private static void doTheTest() {
        try {
            UIManager.setLookAndFeel(new SynthLookAndFeel());
        } catch (UnsupportedLookAndFeelException e) {
            throw new RuntimeException(e);
        }
        
        JTextPane textPane = new JTextPane();
        SynthTextPaneUI ui = new SynthTextPaneUI() {
            @Override
            public void installUI(JComponent c) {
                JTextPane textPane = (JTextPane) c;
                System.out.println("textPane.getCaret() instanceof DefaultCaret = " +
                        (textPane.getCaret() instanceof DefaultCaret));
                System.out.println("textPane.getHighlighter() instanceof DefaultHighlighter = " +
                        (textPane.getHighlighter() instanceof DefaultHighlighter));
                super.installUI(c);
                System.out.println("Method installUI called.");
                System.out.println("textPane.getCaret() instanceof DefaultCaret = " +
                        (textPane.getCaret() instanceof DefaultCaret));
                System.out.println("textPane.getHighlighter() instanceof DefaultHighlighter = " +
                        (textPane.getHighlighter() instanceof DefaultHighlighter));
            }
        };
        textPane.setCaret(CARET_STUB);
        textPane.setHighlighter(HIGHLIGHTER_STUB);

        // the following will lead to expected behavior
// textPane.setCaret(null);
// textPane.setHighlighter(null);

        textPane.setUI(ui);
    }


    private static final Highlighter HIGHLIGHTER_STUB = new Highlighter() {
            public void install(JTextComponent c) { }
            public void deinstall(JTextComponent c) { }
            public void paint(Graphics g) { }
            public Object addHighlight(int p0, int p1, HighlightPainter p) throws BadLocationException {return null;}
            public void removeHighlight(Object tag) { }
            public void removeAllHighlights() { }
            public void changeHighlight(Object tag, int p0, int p1) throws BadLocationException { }
            public Highlight[] getHighlights() { return new Highlight[0]; }
        };
    private static final Caret CARET_STUB = new Caret() {
            public void install(JTextComponent c) { }
            public void deinstall(JTextComponent c) { }
            public void paint(Graphics g) { }
            public void addChangeListener(ChangeListener l) { }
            public void removeChangeListener(ChangeListener l) { }
            public boolean isVisible() { return false; }
            public void setVisible(boolean v) { }
            public boolean isSelectionVisible() { return false; }
            public void setSelectionVisible(boolean v) { }
            public void setMagicCaretPosition(Point p) { }
            public Point getMagicCaretPosition() { return null; }
            public void setBlinkRate(int rate) { }
            public int getBlinkRate() { return 0; }
            public int getDot() { return 0; }
            public int getMark() { return 0; }
            public void setDot(int dot) { }
            public void moveDot(int dot) { }
        };
}
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The output will be:

textPane.getCaret() instanceof DefaultCaret = false
textPane.getHighlighter() instanceof DefaultHighlighter = false
Method installUI called.
textPane.getCaret() instanceof DefaultCaret = false
textPane.getHighlighter() instanceof DefaultHighlighter = false