YIIMP css files are found in several locations depending on what you wish to update.
The “main.css” file can commonly be found at /var/web/yaamp/ui/css/
Changing the colors of the graphs is a bit more challenging as not all the updates are made strictly to css files. Try looking in /var/web/extensions/jqplot. There you will find a file, jqPlotCssStyling.txt that will give you some pointers. Be sure to review jqplot.com for the scoop on how the graphs really work.