This documentation will guide you on How to Insert CSS using Custom HTML code
When on SiteBuilder Dashboard. Hover on Pages option given on the left sidebar and then click on All Pages. Once on all Pages Click on Edit Icon of Any Page.
You will be redirected to Editor.
On SiteBuilder Editor hover on more menu on the navigation bar. When you hover on more menu three buttons will be drop down i.e Reset Page, SEO Options and Custom HTML.
Click on Custom HTML.
On clicking Custom HTML you will see a dialog box will appear. Insert all your CSS under style tag as shown in image below.