Stopping scrolling turning menu white?

Hi there,

when you scroll down the menu goes white, I want it to stay black, how do I do this? Also, How can I change the font across the whole site so it is uniform, finally, how can I keep social media icons when viewed on a phone and also how do I get to add a TikTok icon, it doesn’t seem to be available

thanks!

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