Use something like this:
' Bold toggle
If RichTextBox1.SelectionFont.Bold Then
RichTextBox1.SelectionFont = New Font(RichTextBox1.SelectionFont, RichTextBox1.SelectionFont.Style Xor FontStyle.Bold)
Else
RichTextBox1.SelectionFont = New Font(RichTextBox1.SelectionFont, RichTextBox1.SelectionFont.Style Or FontStyle.Bold)
End If
' Italic toggle
If RichTextBox1.SelectionFont.Italic Then
RichTextBox1.SelectionFont = New Font(RichTextBox1.SelectionFont, RichTextBox1.SelectionFont.Style Xor FontStyle.Italic)
Else
RichTextBox1.SelectionFont = New Font(RichTextBox1.SelectionFont, RichTextBox1.SelectionFont.Style Or FontStyle.Italic)
End If