Generate Own Custom CSS Button with icon

What you are seeing is from a short code, a shortcode that does not exist anymore. Since you’ve changed themes and seeing this, it means that that shortcode is defined in your previous theme’s functions.php

What you’ll need to do is, look at your previous theme’s functions file, and locate where the shortcode is defined. If you have a proper code editor, you can just search for something like add_shortcode. That action will look something like this:

add_shortcode( 'button', 'some_function_name' );
function some_function_name() {}

You need to copy all of that. This needs to go into a functionality plugin. Never add shortcodes into your theme, rather create a plugin and add them in there. This should solve your problem