Web Designer: Textbox / Empty Field Text

The blank text (empty field text) is always displayed in the browser in the standard font color. How can the font color only of the empty field text be changed?
The use of the layout definition "VI_Styles_Textbox_Search" or the definition of a customer-defined layout definition using the class ".imx-search input :: placeholder" did not lead to success.

Version is: 8.1

Thanks a lot, Christian