Update fxa banner, simplify text

![screenshot 2014-09-22 09 37 55](https://cloud.githubusercontent.com/assets/211578/4357908/2cefb2ca-4266-11e4-9795-e8e296121d95.png)
320px wide

![screenshot 2014-09-22 09 38 10](https://cloud.githubusercontent.com/assets/211578/4357909/2cf48de0-4266-11e4-8037-ae3807620d46.png)
700px wide

![screenshot 2014-09-22 09 38 25](https://cloud.githubusercontent.com/assets/211578/4357910/2cfa6f1c-4266-11e4-8b36-7672b165dd2c.png)
>1024px wide
If it's trivial

On Sep 22, 2014, at 1:41 PM, Mark Striemer <notifications@github.com> wrote:

> In src/media/css/site-banner.styl:
> 
> >      }
> >  
> >      a {
> > -        color: #fff;
> > -        font-weight: 500;
> > -        text-decoration: underline;
> > +        $bg-color = #118E36;
> > +        background-color: $bg-color;
> > +
> > +        &:hover {
> > +            background-color: darken($bg-color, 10%);
> > +        }
> > +    }
> > +}
> > +
> > +@media (min-width: 1025px) {
> > +    .mkt-banner.mkt-banner-success .mkt-banner-content {
> You want a less specific selector?
> 
> —
> Reply to this email directly or view it on GitHub.
>