Adding style to a watermark
WatermarkCssClass – The CSS class to apply to the TextBox when it has no value (e.g. the watermark text is shown).
TextBox also has CssClass property to attach a CSS class to it for styling but WatermarkCssClass only apply styles when TextBox is empty. Following are an example CSS class used :
border: solid 1px #808080;
You can add the above CSS classes into an external CSS file and can attach the CSS file with ASP.NET page using link tag.