When you set BackColor to the TextBox, it actually overwrite the default TextBox style. In order to restore to original style you can set the style manually.
Set BorderColor, BorderWidth, BackColor. You can also use css style to do this by just changing the css class name.TextBox1.Enabled = true;
TextBox1.BorderWidth = new Unit(1);
TextBox1.BorderColor = System.Drawing.Color.LightGray;
TextBox1.BackColor = System.Drawing.Color.White;
Hope it helps!
Don't forget to click "Mark as Answer" on the post that helped you.
This credits that member, earns you a point and marks your thread as Resolved.