Internal Server Error, also the 500 Error, can be caused by something from your web's server, and it is not related to the theme. There's not much you can do about that. But following are some possible ways for you to fix this issue:
1. Increase the PHP memory limit:
One way is to increase the memory limit allocated to PHP. This setting increases PHP Memory only for WordPress, not other applications.
By default, WordPress will allocate 40MB for a single site and 64MB for multiple sites. This code is found at the beginning of /wp-includes/default-constants.php), so the setting in wp-config.php should be adjusted to be higher than 40MB or 64MB depending on your setup.
For more specific instructions on this, you can refer to https://codex.wordpress.org/Editing_wp-config.php#Increasing_memory_allocated_to_PHP
Note that some host might not allow users to increase the memory limit of PHP. In this case, you might have to contact your host company and ask them to increase the memory limit for your site.
2. Adjust host configurations:
Open file "php.ini", which is located in host (contact with your host service provider to help you with it). Then find and customize the following fields:
- upload_max_filesize = 32M
- post_max_size = 48M
- memory_limit = 128M
- max_execution_time = 600
- max_input_vars = 10000
- max_input_time = 400
3. Clear browser's cache / cookies:
Sometimes clearing cache or browser cookies can help you, too. There might be something from your browser that causes the problem. Just delete caches / cookies then refresh your browser. You can rest assured because this won't affect your database.
In case you need more guides on this issue for reference, pay a visit to: http://pcsupport.about.com/od/findbyerrormessage/a/500servererror.htm