change text&color when text is highlighted

this is my source that i created

Private Sub FontToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles FontToolStripMenuItem.Click
        FontDialog1.ShowDialog()
        RichTextBox1.Font = FontDialog1.Font
    End Sub


    Private Sub ColorToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ColorToolStripMenuItem.Click
        ColorDialog1.ShowDialog()
        RichTextBox1.ForeColor = ColorDialog1.Color
    End Sub

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when  i wanna change color and font in richtextbox,all of text in textbox is changed.how to make source code that change color and font in text that we highlight