Change Slider Text Colour in Mobile displays

Hi When looking at a computer screen the text colour appears white on the slider image, which is good as it is readable. Although the text on the mobile display does not appear on the image rather on the white background and therefore is lost. You can partially see it although loses any value or impact with white on white. How can I change Slider Text Colour in Mobile displays so that when it appears below the image it is then readable, e.g. black on white background? Thanks and appreciate your help with this Derek