@wtb295 welcome to the forum.
I do not use the Rich-Text editor but I had a look and it seems that if you put the below css into userstyle.css the checked checkboxes will no longer fade.
li.checked {
opacity: 1 !important;
/* sets the opacity of the checked checkbox text*/
}
"Before" and "After" screengrabs
If you have not tried custom css yet, this wiki page should give you an introduction.