The Sass Theme Builder tool enables you to upload your own theme that you have previously created. You just need to select the Import existing theme option.
You must upload the primitives.scss file or both ScssPrimitives.json and UsedControls.json files if you had used the previos version of the Theme Builder.
After you create the custom skin you will need to register it on the page where it will be used.
Once you have uploaded the theme you can customize it as described in the create and download article.