Ive upgraded from EE 2.1.1 to 2.1.2 OK now (having had problems as detailed hand fixed here - http://ellislab.com/forums/viewthread/176081/#835515 )
I then went about upgrading to 2.1.3 and hit a problem when I uploaded the backed up /themes folder, getting the following error o the CP pages:
A PHP Error was encountered
Severity: Notice
Message: Undefined variable: theme_css_mtime
Filename: _shared/header.php
Line Number: 9
index.php?S=dfeec82a25d6f33b3ceee01090c21c8f6bca1db0&D=cp&C=css&theme=default&v=" type="text/css" media="screen" title="Global Styles" charset="utf-8" >with the page all messed up having no styling attached to it.
I then went back and replaced the themes folder with the default ee2.1.3 base install /themes folder and the css was working (altho some of the JS wasn’t working such as the accessories windows and the CP navigation dropdowns)
When I tried to re-upload the backed up /themes folder the same error occurred.
I’m aware that this may therefore be to do with some 3rd party addons that have messed things up, but any pointers would be greatly appreciated right now!!