It took me awhile to get the epub style sheet right so that the formatting looks good. I used
CoolReader 3 for desktop to go back and forth to see what each change to the EPUB stylesheet would look like. I finally got it to where I want it.
You just put the epub.css in the crengine folder on your reader once your done editing it and make sure in settings that it's set to override the book's style.
I use BookDesigner 5 to make my source HTM file before converting it into Calibre to make a final epub file. BookDesigner 5 makes it easy to get good formatting in CR3 with Epub.
The style sheets are found inside the Cool Reader 3 for windows installation zip.
They can be edited with notepad or any plain text editor of your choice like PSPad or Notepad++