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 the Theme Builder 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 them available for your site visitors: