In this article i will describe how to upload custom CSS to site collection in office365.

Først go to your site collection — go to site settings — find the link “site collection features”.

Activate feature called “SharePoint Server Publishing Infrastructure”.

Now go to site settings — SharePoint Server Publishing

Now you can go to site settings and find link “master page”.

If you want to upload your CSS to global must create using SPFX Application Customizers.