change block icons and nav font color

Hi everybody,
I am working at my first website and I am stuck in this two trubles, I hope you guys can help me out!
1) I edited my css to change the nav background to white and its font to be black, however something in the style.css is more specific than my command and all the visited links turn white so it looks like they aren’t there. Can anybody tell me how to fix this? how my selector could be more specific?

2)I need to change the 4 block icons(under the main picture) using images that I uploaded in my library. the theme allows me to chose between standard ones and I have no clue about what to write and in which php file.
my theme is asteria lite(the free version) and my terrible website is this:
https://ylenia.aryma.net/wordpress/
Please help me!