Change Colour of Drop Down

Hello,

I've just added a new page as a drop down from a page, but how do I change the colour of the drop down and text please?

See here: https://ibb.co/kmW7ZQ8

On mobile the drop down shows as white text on grey so it's readable but on desktop it's almost grey on grey - not sure why it's different or how to update it on desktop only?

Thanks,
Ella