This page consists of screenshots from the Layout Settings – Custom CSS Screen in Coming Soon Booster WordPress Plugin.
This gives an idea about how to add CSS.
Custom CSS : In this field, you would need to provide CSS code manually to add extra styling to the layout of Coming Soon Page. This CSS style will override the plugins css and provide you the ability to customize the basic layout of coming soon page.
Once everything is being setup, you would need to click on “Save Changes” button located at the bottom of the page to save the changes.