As SAP PD 2.0 uses Webdynpro navigation and not Portal navigation, so the user personalization of themes are not supported for newly opened window sessions like Idea Editor, concept Editor etc.
Problem can be solved by below mentioned steps:
a) Go to Visual Administrator > Server > Services > Configuration Adapter > webdynpro > sap.com > tc~wd~dispwda > propertysheet default.
b) Change the property 'sap.theme.default' to the theme that is desired
to be set.
For example to set the default theme to 'SAP Chrome', add the following value:
http://<hostname>:<port>/webdynpro/resources/sap.com/tc~wd~dispwda//global/SSR/themes/sap_chrome
However, this solution will apply globally and will change the default theme for all users.