Momen ,
Can you try loading the custom theme via ?<parameter>=<value>
<parameter> = sap-theme
<value> = custom theme name
Make sure you have already performed the below steps and able to access the theme rootpath via https and web dispatcher port.
https://<hostname>:< web dispatcher port>/sap/public/bc/themes/~client-<client>
Note
The value of this parameter contains the ID of your theme and the theme root:
sap-theme=<theme ID>@<theme root>.
- The theme ID is the ID that was entered in the Publish dialog box.
- The theme root is the URL of the theme repository that contains the theming files.The theme root path looks as follows:https://<hostname>:<port>/sap/public/bc/themes/~client-<client>You can only specify the URL of the host you are using to run the UI theme designer. Loading a theme from a remote host is not yet supported.
Reference : Setting a Custom Theme via URL Parameter - UI Theme Designer - SAP Library
Also , make sure that the webdispatcher configuration was good for HTTPS and the /sap/public/bc/ path was added there , since theme root uses the same path.
Regards
Virinchy.