Hello Vij,
Sorry, don't know what I was thinking that day.... It is not logo itself, but logo text, right. 
What HongYan talked about, it is used in non-IC agent. I checked a system with ehp2 using IC_AGENT role, it should be class ctx_branding_message.
The way I used is:
1.Click on F12 from the webui page, the developer tool will start. Then click on the 'arrow' icon.
2. after clicking on the 'arrow' icon, click on the logon text 'interaction center', it will be highlighted with blue square
3. Then in the Developer tool, its source will also be highlighted. We can see its class is 'ctx_branding_message'. From the right side, we can see its size definition.
4. I changed the size definition, the logo text is also changed accordingly. There is a hyperlink on the right to the size definition, click it.
5. it will show the full path of the css file
6. Then go to se80, we can find it.
