The CSS source files are located in /cp-styles

When compiled, the minified CSS file lives here:
/themes/ee/cp/css/common.min.css

The source files for the current JS live here:
/themes/ee/cp/js/src/

When compiled, the processed JS files live here:
/themes/ee/cp/js/build/

NOTE: These files are NOT minified. You can read more about this in "Running build system".