Change Navigation Search Transparency and Font colour

I am using the navigation search within the primary navigation. My background image is quite dark so it's difficult to see what term is entered into the box with the black font and semi-transparent set up on the navigation search. Making the search box completely white would be the best option. Therefore, is it possible to alter some settings - i.e. box colour, transparency, font colour - making the search functionality more usable?

Note I found the below CSS on another post and put it to the Additional CSS section of Customize. However, it didn't change anything; perhaps I put the CSS in the wrong place (I'm not really a developer, just having to make a few tweaks myself as my usual developer isn't available).

.navigation-search input {
opacity: 1;
}