Hi designedbyash,
If you like to edit code then all the css file is from theme folder(interface->style.css) and if you like customization in code then the header section is from theme folder (interface-> inc -> structure -> header-extension.php on line no 305 to 313 ). All the code is inside this header-extension.php. You need to look over it.
While customizing the code first of all you need to create a child theme and then customize the code. So that while updating to new version you customized code will not be lost. If you make changes in the code directly then each and every time while updating then you need to customize the same thing.
Thank you!