The Sass Theme Builder tool provides you with the ability to upload and import a theme that you previously created,so you can modify it further. To get started, open the Sass ThemeBuilder click on Import Theme option, then click on Select Files.
You must upload the variables.scss file.
If you had used the previous version of the Theme Builder, you must upload both ScssPrimitives.json and UsedControls.json files
Once you have uploaded the theme you can customize it as described in the Create article.