Fix compatibility with minimal mode

Hi,
with roundcube 0.9.0 and Firefox 21.0, the `#mainscreen {...}` rule defined in headermini.css gets overriden by the `.minimal #mainscreen {...}` rule defined in styles.css. Thus there is a huge amount of space available between the topbar and the toolbar.
I fixed that and made the logout button visible.

Thanks for your plugin! Apart from that little issue, it was exactly what I needed.