With so many textboxes, I would create a derived TextBox control like this:
Public Class SuperTextBox
Inherits System.Windows.Forms.TextBox
Protected Overrides Sub OnGotFocus(ByVal e As System.EventArgs)
Me.BackColor = Color.GreenYellow
MyBase.OnGotFocus(e)
End Sub
Protected Overrides Sub OnLostFocus(ByVal e As System.EventArgs)
MyBase.OnLostFocus(e)
Me.BackColor = SystemColors.Window
End Sub
End Class
Then I would change all the textbox's types (in the designer file) from System.Windows.Forms.TextBox to the derived SuperTextBox class