Sometimes theme authors ask you to add code in functions.php and you can use that.
There is another option. You can use a so called Must-Use plugin which doesn't have to be activated.
WordPress runs it automatically. It will show up under Plugins > Must Use

I have created a sample plugin that you can use to add extra functionality.
https://raw.github.com/orbisius/orbisius-site-util/master/orbisius_site_util.php

How to install it?
1. Create a folder called wp-content/mu-plugins/ (if it doesn't exist yet)
2. Save/upload the file in as wp-content/mu-plugins/
3. Add some functions in the function called 'orbisius_site_util_after_setup_theme'
4. Done.

Troubleshooting

Note: if you make a php error ... your site will crash. Don't panic. Try to see if there is a closing bracket or a semi-colon missing.

 

Did your WordPress site ever crash because bad plugins or themes? Next time you should use qSandbox.

Disclaimer: The content in this post is for educational purposes only. Always remember to take a backup before doing any of the suggested steps just to be on the safe side.
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