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: