Messed up backend - Joomla

Possible error message:
Couldn't create the required cache dir: …/media/nextend/cache/… Please make sure that the folder writeable by PHP!

If your backend looks messed up, the css codes doesn't seem to load, that could happen for two reasons. One if the files weren't created with the right permission. You can fix that by going to the SystemGlobal ConfigurationServerenable the FTP and give the connection details. After that install the Accordion Menu again, and like this the permission of the files will be different.

The other reason why this could happen, if the media/nextend/cache folder isn't writeable by php. Change it's permissions with everything in it to 755,775 or 777 in this order, and based on the server settings one of them will work. You can do this for example with Filezilla, just connect to your ftp with it, go to the media/nextend/ folder, right click on the cache folder and choose File permissions…
change the Numeric value to 777, and check the Recurse into subdirectories too.
If you are worrying about the 777 permissions, read this part of the documentation.

Also your backend will look like this, if you turned on the Gzip compression in the ExtensionsPlugin ManagerNextend Library, and your server (like most servers) doesn't support it. Firebug with Firefox browser is the best tool to work on website issues, so you should install it. After that you can fix this issue by going back to the Nextend Library, right click on the Gzip compression, and select Inspect Element with Firebug.

Open up the website's code under it, until you find this:

<input id="jformparamsconfiggzip" type="hidden" autocomplete="off" value="1" name="jform[params][config][gzip]">

Change the value to 0, and click on Save. This way the Gzip compression will be turned off again.