During installation of MediaWiki 1.25beta (and 1.26alpha) with Vector skin installed (this one in git branch REL1_25), the CSS styles are not loaded: the file /mw-config/index.php?css=1 return an HTTP error 500 (PHP/5.5.6-1 + php-fpm + nginx/1.4.4 -- under Linux Mint LMDE). In nginx log, there is:
```
2015/05/04 19:40:12 [error] 20033#0: *630 FastCGI sent in stderr: "PHP message: PHP Fatal error: Call to a member function readStyleFiles() on a non-object in [root path]/includes/installer/WebInstallerOutput.php on line 169" while reading response header from upstream, client: ::ffff:127.0.0.1, server: localhost, request: "GET /mw-config/index.php?css=1 HTTP/1.1", upstream: "fastcgi://unix:/var/run/php5-fpm.sock:", host: "localhost", referrer: "http://localhost/mw-config/index.php"
```
When I change the git branch of Vector skin to REL1_24, it works fine. So I guess the installer is not converted to the new extension registration, which came into effect with Vector REL1_25.