I figured it out myself.
1. thtmlb_stand.css file is cached by web server along with all other CSS files. Hence, changes to these files will reflect only after server restart.
If you want to see changes immediately, you need to set 'Expiration Time Client Cache' of thtmlb_stand.css file.
In object navigator -> right click on css file -> 'Expiration Time Client Cache' -> check 'Expiry Active' and 'Individual Expiry Date' and set seconds/min/hrs/days to zero.
That's it. Now changes made to CSS will be reflected on WebUI immediately after browser refresh.
2. To change header, you have to locate corresponding BSP application. This can be found at location ->
"spro -> img -> crm -> ui framework ->tech role def. ->define layout comp"
In above list, search for 'header' component.
3. Same goes with footer, there must be some BSP application (either standard or custom) that you need to F4 search in SE80.
