You are definitely on the right track, it just comes down to where to look. The nav is definitely complicated to sort out but it is doable. First off, here is a link to an answer I gave recently regarding how to target the last item in the navigation. This doesn't directly answer everything you need in your question so I will dig a bit more into your question.
Below are some CSS examples for how to change text color, hover colors and selected link colors (I have switched to the Clean Lines theme to test)
/*text color of all links*/
#navigation li a {
color: red;
}
/*background color of active link*/
#navigation li#active {
background-color: black;
}
/*background color of hovered link*/
#navigation li:hover {
background-color: palegreen;
}
/*color of hovereded link text*/
#navigation li:hover a {
color: black;
}
/*text color of active link*/
#navigation li#active a {
color: white;
}As for the question of the lines of full width, I too had them disappear. It looks like when you toggle the option to full width (Theme > Theme Options) it omits them by default. You can add them back in yourself by using
#navigation:before, #navigation:after {
display: inline-block !important;
}From here it's trivial to then change the color
#navigation:before, #navigation:after {
background-color: red;
display: inline-block !important;
}Hopefully this covers everything you were looking for. Let me know if you need any more guidance. I will agree that customizability is very much the responsibility of the user and their external knowledge as the basic options provided do leave a bit to be desired. I don't know the browser you use (I favor Chrome for work) but each come with a variant of "Developer Tools" which can be accessed by pressing F12. From here you can poke around your html and select elements on the page if you're curious about how to access them by id, tag, class, etc. Another way is to hover over something on the webpage, right click and choose "Inspect" which will pull up the Dev Tools right onto that object. I don't know what "some experience with CSS" means so I'm adding all of this in the event you would find it useful in the future. I don't mean to insult your ignorance if that is not the case 
For my examples I stick them within the global footer typically from within the settings (Settings > SEO). Long term or with more CSS you can always put these at the theme level styles (Theme > Edit HTML/CSS).
You are definitely on the right track, it just comes down to where to look. The nav is definitely complicated to sort out but it is doable. First off, here is a link to an answer I gave recently regarding how to target the last item in the navigation. This doesn't directly answer everything you need in your question so I will dig a bit more into your question.
Below are some CSS examples for how to change text color, hover colors and selected link colors (I have switched to the Clean Lines theme to test)
/*text color of all links*/
#navigation li a {
color: red;
}
/*background color of active link*/
#navigation li#active {
background-color: black;
}
/*background color of hovered link*/
#navigation li:hover {
background-color: palegreen;
}
/*color of hovereded link text*/
#navigation li:hover a {
color: black;
}
/*text color of active link*/
#navigation li#active a {
color: white;
}As for the question of the lines of full width, I too had them disappear. It looks like when you toggle the option to full width (Theme > Theme Options) it omits them by default. You can add them back in yourself by using
#navigation:before, #navigation:after {
display: inline-block !important;
}From here it's trivial to then change the color
#navigation:before, #navigation:after {
background-color: red;
display: inline-block !important;
}Hopefully this covers everything you were looking for. Let me know if you need any more guidance. I will agree that customizability is very much the responsibility of the user and their external knowledge as the basic options provided do leave a bit to be desired. I don't know the browser you use (I favor Chrome for work) but each come with a variant of "Developer Tools" which can be accessed by pressing F12. From here you can poke around your html and select elements on the page if you're curious about how to access them by id, tag, class, etc. Another way is to hover over something on the webpage, right click and choose "Inspect" which will pull up the Dev Tools right onto that object. I don't know what "some experience with CSS" means so I'm adding all of this in the event you would find it useful in the future. I don't mean to insult your ignorance if that is not the case
For my examples I stick them within the global footer typically from within the settings (Settings > SEO). Long term or with more CSS you can always put these at the theme level styles (Theme > Edit HTML/CSS).
