J3.x:Backend Login looks bad
From Joomla! Documentation
In Joomla 3.4.2 we improved the styling of the backend login page.
Backend login has white background or the login button don't look correct.
What is the cause
Updates to the Isis template's CSS were made and the browser's cache has not been properly cleaned.
How to fix
Refresh the page and clean your Browser and Joomla Cache