I also got an Internal Server Error. And, I tried many things like you did: .htaccess, plugins, themes, etc. Without success.
Eventually I contacted the hosting provider to see the server error log. And, the message was “PHP Fatal error: Allowed memory size of 41943040 bytes exhausted …”
The problem was solved by adding a line to the wp-config.php file:
define(‘WP_MEMORY_LIMIT’, ’64M’);
Perhaps you have the same problem. Otherwise, you need to contact the web host.
I would suggest you to create a child theme to do a modification. According to http://codex.ww.wp.xz.cn/Child_Themes: “Child themes are the recommended way of modifying an existing theme.”
I am new to WordPress. I have tried a few modifications to several themes, and never encountered problems. So far, I find Customizr is the most customizable theme.
To debug a web page, it would be much easier if you use Firebug, or the newly created Firefox Developer Edition. Both of them are free, and good stuff!
