While it is quite possible to change the various html/css/js files included in PI Coresight, I have to let you know this is not a supported feature. In particular, there will quite likely be issues when you upgrade to a newer version of the product. I would therefore strongly recommend against doing such edits.
The next version of PI Vision (the new name for PI Coresight) will include some more styling options and more styling options are being requested and considered.
Would you be able to register your request on the UserVoice site? This would make the development team aware of what type of styling of PI Vision is most interesting to users.