how to change website attributes with a drop down menu?

I’m a high schooler who’s taking software engineering and I know basic HTML, I’m trying to make a website and need help with a specific feature.

I’ve made a drop-down menu that changes the background and font color of the page. It changes light mode dark mode and blue mode. The thing is the js to change the font only works on black font and not any of the other colored fonts on my page and I can’t figure out how to also force the color of the navbar to change depending on the dropdown, the color stays the same.

How do I use js to change the style of colored fonts and a change the color of the navbar with my dropdown menu?

I can provide a link to the website and the specific code I’ve used if needed

That would be very helpful