Learning how to target your site’s CSS will help you make certain design changes. Here are some very helpful posts that will help you customize your site with CSS:
https://dailypost.wordpress.com/2013/07/25/css-selectors/
https://dailypost.wordpress.com/2013/06/21/css-intro/
https://dailypost.wordpress.com/2013/08/29/css-matched-rule-pane/
If CSS is new to you, here are some resources for learning more about it:
Using Firebug, I found the class for your current/hover menu background and text colour:
#nav .current_page_item a, #nav li:hover > a, #nav ul ul *:hover > a {
background-color: #xxxxxx;
color: #xxxxxx;
}
For non-current/hover menu items, the style is:
#nav a {
background-color: #xxxxxx;
color: #xxxxxx;
}
As you’ve already declared !important on some of your more basic link colours, those will override the text colour above. For example, you’ve already added this to your custom CSS:
a {
color: #257BBC !important;
}
It’s a good practice with CSS to be as specific as possible with declarations, and not to use !important unless there is no other alternative.