How to find and import default CSS tags from PHP file

I have a report generation page, that is PHP based, but I can’t use basic CSS classes in it for some reasons. For example, the i-column-header class doesn’t do anything. I am Using PhpStorm and if I go into a CSS file, I see that it has the classes, and it says “from html”.

I am not sure where the html is located, or how to reference it from my PHP file. I am OK with just having a CSS file with all of those default classes hanging around.

Can anyone please tell me where to find the classes, or if there is a resource where they are located. I have tried looking this up, but couldn’t find it anywhere.

Answer

I have checked the classes requested and they work ok in the PHP files.
But a word of caution for the future people looking at this Question and Answer

Do not use CSS files, as they don’t really work well with PHP. When defining your own classes, define them in the PHP file, or an HTML file with a <style> tag.