Setting up an online estimating module requires the insertion of php code and the loading of a style sheet (css) this would require to change the main theme, which is absolutely not recommended (data lost in case of update).
i installed the X child on my site https://berton-groupe.com.
I want to insert 4 css files + php code in the page where i want to set up the estimating module.
whow can i insert the css files in the child theme please ?

