J3.x: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
Backend login has white background or the login button don't look correct.
Versions affected
General Information
This pertains only to Joomla! version(s):- 3.4.2+
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