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Previewing Style Parameters 
Before creating your own theme, select one of the templates provided by SAP and look at the pre-defined parameters. Check what you want to change.
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1. Start the Theme Editor. To do so choose System Administration ® Portal Display ® Theme Editor in the administrator role.
2.
Click on the initial screen of the
required template.
You are in edit mode in the Theme Editor.
3. Click on the individual elements on the left side and look at their parameters in the right preview window.
You can look at the specific properties for each theme element of the user interface (for example Portal Masthead, Portal Body).

The nonProxyHost settings might be the reason if the preview does not work. See also Checking the nonProxyHost Settings for the Theme Editor.