Backend Login looks bad
From Joomla! Documentation
Joomla!
3.4.2
In Joomla 3.4.2 we improved the styling of the backend login page.
Errors reported[edit]
Backend login has white background or the login button don't look correct.
Versions affected[edit]
General Information
This pertains only to Joomla! version(s):- 3.4.2+
What is the cause[edit]
Updates to the Isis template's CSS were made and the browser's cache has not been properly cleaned.
How to fix[edit]
Refresh the page and clean your Browser and Joomla Cache