The style sheet is PSSTYLEDEF_SWAN.
You can find in:
Home > Peopletools > Utilities > Administration > People Tools Options.
However, the above bar which is generally in blue and have PeopleSoft written in it is actually an image which is repeated in such say that you see that bar across the page.
Let me see if i can find the style class used for the same.
Also, if you want to find the class...you can find the class name and the other details in the html code of the page....
How it works? ->
1. Open a page in Peoplesoft.
2. View the HTML.
If you use Firefox or Chrome, you can select only a section/portion of the page and right click to view source. That will give you the HTML code only for that portion.
Personally, i prefer Firefox because i am used to it. :) Chrome is better and gives more information than firefox but i guess it is way too much for me.
IE sucks for such things. IE can give us the HTML for entire page only which makes it difficult to understand when the user (like us) is not an HTML expert.
I will let you know in case i get anything on the style class and the image.
Oh...Subhash provided the image name PT_HEADERBG_SWAN. I missed that reply. You need to change that image. That image is repeated by PeopleSoft to make that entire bar in the header on the top of any PS page.