After you’ve made your style changes, re-zip all the files from the theme you downloaded, even the ones you did not edit. Zip the files following the original structure—if you change directories or zip them differently, you will not be able to upload your customized theme.
This is the correct structure for a theme zipfile.
Make sure you create the zipfile with the directory structure as shown in the image above. It is an easy mistake to include the containing directory in the zipfile. If you do this the theme will not work. Re-zip the theme correctly and update in your Live Forms account.
Uploading the Re-zipped Files
In Live Forms, choose Themes from the left-hand menu and then click the icon in the upper right corner. Give your theme a name and description, browse to the zipped file and click Upload.
Do not use special characters (such as '''/''', ''':''', or '''&''') in the theme name. These characters can cause problems when you upload or download themes.