Read "How to Use" Guide
Select Your Parent theme from LUMA or BLANK
Upload Mockup of your website in PNG or JPG format from "Upload Design" button to get color codes.
Enter value in textboxes as per your requirement. You can use px, em or any format.
Download theme.less file from "Download Theme file" button.
Make sure to place generated theme file under app/design/frontend/Vendor Name/Theme Name/web/css/source/ folder.