Css styling for blog title and menu bar

Hi

I use the following css insert to set a light weight on my blog title (the theme only allows for normal or bold)

#header a {
font-weight: 300 !important;
}

This works, but it also forces 300 weight on the text in the header menu bar, which I don't want.

Can you tell me, please, how can I style the blog title and menu bar fonts separately? (Again, the theme doesn't allow weight setting for menu bar fonts, only px & font.)

Best, Kit