you "uploaded customSh.css in it's theme's files", that is a bad idea. Never put your own files in this directory. They will change with every patchset.
Instead you could open the Theme Roller from the APEX Developer Toolbar and put your css modification as custom css.
the theme roller is good enough for me to write my own CSS. However I would like to upload CSS files, "font-awesome-animation.min.css" to be precise, in my app in order to exploit them. I could copy past their content in the Theme Roller window but that would be messy, rude and ugly. So the reasons change but the question stay:
I would like to know how to upload and reference my CSS files in Theme42.
You can add custom CSS files to the user interface via:
Shared Components -> User Interface Attributes -> select the Desktop interface -> select the Cascading Style Sheets tab