weather-screen-green-narrow.css looks like the CSS you're using. In there, look for ####Left Side Bar####. That's where I was able to change my menubar width, however I only added 6 pixels to it.
I don't understand something though....you have the theme switcher set to "false", but when I'm on your site, I'm able to switch the colors. If you want to stick with the green narrow layout, turn that color option off also and your changes in the CSS file should take place.
Forgive me....I'm really not good at this PHP stuff. I'm just feeling my way through it LOL