Hi Saket,
Please make sure you are at the latest UI version 1.28.11
Also, please implement below SAP Notes to fix the issue
- 2163728 - UI theme designer for ABAP: Version 1.8.2
- 2159897 - UI theme designer for ABAP: Images not saved to server
Reference Link: https://scn.sap.com/thread/3743977
Regards,
Vijay