Drop shadow on menu header link colour

Hi,
is it possible to drop shadow on header link ?
I have Header background image witch is colorful, dark on the outside, and light in the middle. The menu on the PC looks fine, but the text on the menu is not visible on the mobile phone. Most text is on the dark, but also fall on the light background. Therefore, I want to put a shadow so that the same text appears on both a light and a dark background.
Is there a possibility to do this with Additional CSS code in Storefront ?

The page I need help with: [log in to see the link]