How to stretch Main Menu to fill entire bar

Yes, I know this is a common topic.

I started working from this post here that suggested editing CSS code by adding the following script:

@media only screen and (max-width: 1350px) {
.top-bar-wrap, .header-elements-wrap, {
width: 100%;
}
}

I believe this was suggested for the root files of the theme however. I am using the free version of the JustWrite theme currently along with the Simple Custom CSS plugin as I do not have direct access to the server, and would also prefer not to mess with the source code.

Long story short, I put that code in (both forms from the thread) and no dice. You can see the problem here – the menu bar doesn’t fill the whole space and looks a bit silly.

What I noticed when putting the code into the plugin was that the word “only” (line 1) came up grey, like comments do, as if it were not recognised. I’m thinking there’s either a problem with the plugin or the script is outdated, the source thread is a year old (the others I see are even older).

So, do the wise code wizards have any suggestions?

Side note: is there a CSS/WordPress guide for newbies? I don’t have any experience in this area, and I’m fairly new to WordPress.