Fix login button styling

I fixed the login button for the pages still using the old style which was also causing whitespace to appear below the navbar. It was reported [here] (http://forums.hummingbird.me/t/login-or-register-button-before-logging-in-looks-a-bit-weird/9226) a few weeks ago. In addition, I changed the colour of the mobile menu icon + text to match the theme.