I just did a fresh installation of MediaWiki and used the new 1.26.0. The installer didn't look the way I was used to, but everything went smoothly and there were no errors or warnings.
Once LocalSettings.php was created, I logged in and everything still looked wrong; almost as though there were no CSS being loaded, but not quite. I checked via Firebug and CSS is indeed being loaded.
I went into the preferences and tried the other three themes and they all displayed correctly. I am using a Firefox browser on Mint with a freshly-cleared cache, in case it is some kindof browser issue.
I am attaching a screenshot of how Vector currently looks for me.
Here's the error that the console shows: