According to the Release note from SAP , 7.3 SPS10 got the new UI Theme Designer is available to use
To Enable the feature , it has to be done at the NWA admin page.
Here is the steps to enable the feature.
Login to NWA
Navigate to Configuration -> Infrastructure -> Application Module
search for com.sap.portal.themes.lafservice
Change the Parameter “Determine what will be the theme runtime provider” Value to “LESS
Save and Restart the System
Under Content Administration –> Portal Display : New UI Theme Designer will appear
Notes and Known Limitations
Note 1890375 – Known Issues of UI Theme Designer for SAP NetWeaver Portal.
Note 1927550 – Update UI Theme Designer to version 1.1.4 in the portal.
Note 1895989 – UI5 iView stylesheet rendering issues.
Note 1959708 – Supporting integration of UI Theme designer and UI5 application in the portal.