Something looks a bit odd about your configuration (which is a bit hard to read due to the inconsistent indentation) of your Apache config. You have:
Code:
ScriptAlias /git "/var/www/gitweb/"
But it seems to me that there should be things other than CGI scripts in /var/www/gitweb. The configuration given
on HowTo Forge looks a little bit different.
Also, on Debian 8, the Allow/Deny syntax has changed as Apache 2.4 is in use instead of 2.2. You have the old style syntax in your configuration. I doubt it's your main problem, but it may be causing some side effects.
I've found that the best thing to do is try to load a css file manually in your browser and then check the Apache error_log to see the exact problem. This should give you some clues as to what needs to be fixed.