I need to put logo and client name (image) as shown in the attached screen-shot.
good, now you need to debug it to identify the elements you want to change. I would ensure the area you have marked is indeed part of the theme, and is not rendered by the application self.
The blog suggests modifying standard theme. Due to some client restrictions, I can't change standard theme. How to create custom theme. Moreover , I would have to upload logo(image) in theme though not sure where to upload it using the manual CSS change approach.
This could be a problem, I have absolute no expertise on this, but I also didnt found any possibility to give a custom name while import. In the portal its reverse: you cant replace SAP standard theme stuff at all. But I have an idea: there is more than one ABAP theme, so if your customer says you are not allowed to modify the "standard" ABAP theme they are using, he probably means only one of the available themes, right? Maybe you can choose another one they not use, change it according your needs and apply to your application?
cheers