The 500 internal server error is not a Theme-specific error.
It is a general server error that may be related to your server settings or your WordPress files.
Please check the next video guide to find out where is the problem:
You probably have a problem with the plugin incompatibility. So I suggest you go to your server where the site is hosted (You will need an FTP host, user & pass details from your server.) and rename the plugins folder, located in wp-content / plugins/, by renaming plugins folder on your server, you will deactivate all plugins. Then your website should work.
After that, you will be able to login to your WP dashboard. Once when you logged in, go to Dashboard->Plugins and Activate the plugins one by one until the error occurs again, so on that way you will find what plugin causes the error, then go to the server and delete it.
The next step is to replace a problematic plugin with someone similar or let us know which plug-in made the problem, so we can try to fix the incompatibility.
If the above procedure is too complicated for you, contact your hosting provider support. they can find the problem for you.
Or, give me an Address to your website, WordPress login credentials and your FTP Server login credentials, so I can check this for you.