With Netweaver Portal 7.30 SP10 or 7.31 SP09 SAP launched the new UI Theme Designer. By default the old editor is still loaded, so here are the tips:
1) Enable UI Theme Designer in the portal
In Netweaver Administrator (/nwa) go to Configuration > Infrastructure > Application Modules;
Select the web module "com.sap.portal.themes.lafservice" in the Module List;
Now in the Portal Service Details the Properties are loaded. Change "Determine what will be the theme runtime provider" to LESS.
Save it and restart the portal.
2) By enabling the LESS functionality the portal has added a new menu option under Content Administration > Portal Display > Portal Themes. It is called UI Theme Designer.
If you launch the UI Theme Designer, be sure you are running a modern browser (e.g. CSS3 support -> IE9 or higher, Firefox and Chrome).
Otherwise it won't start / work correctly.
3) Check out the following: Note 1839261 - Application displayed without theme (UI Theme Designer). In our case we needed a patch for SAP_BASIS (SAPKB73108).
4) Now you can run the UI Theme Designer from the portal.
Some interesting information (thanks Shani and Itzik again )
OSS note 1890375
How to... UI Theme Designer - Installation (you are here)