Advanced WebMan Design themes improve performance of your website by creating a single CSS stylesheet. This stylesheet is generated from your theme customizations and is stored in WordPress uploads folder.
For the theme to properly save the file you need to have proper file permissions set up for your WordPress uploads folder. This is usually done by default, but from case to case you might need to check them up, here is how:
- How to Fix “Upload: Failed to Write File to Disk” Error in WordPress article
- Changing File Permissions in WordPress Codex