Is it possible to use a different style and color (per the client needs for example our client needs Yellow instead of dark Blue) other than the OOTB User Interface?
Yes, you can create a custom style. The instructions are in the Administrator's Guide. This is the online help. It is under Administration -> Managing the User Interface -> Customize or Create a Style.
You can create a new style and then define your own stylesheet to set your own colours and layouts.
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