If you have some Custom CSS that you need to apply for your SharePoint or Office365 site, you can easily do this using the Custom CSS feature in the Site Branding & Customization Tool by following these steps.

Step 1: Open the Site Customization Tool

Go to Site Settings

Then to ShortPoint Dashboard

Click on the Site Customizations tile

Then Click on the Customize my site button

Step 2: Apply your Custom CSS

Once the branding & customization tool is opened, Navigate into Utilities 

Then to Custom CSS

paste your CSS Customizations into the Custom CSS Field, and click Apply

And of course, do not forget to Publish your changes in order to make then available for your site visitors