If your custom CSS doesn't appear in the editor, it could be because you typed something wrong, or the CSS file is not loading for some reason.
Try to use CTRL+SHIFT+R to force a reload and emptying of cache.
But then it is also a matter of specificity which rules will take precedence. For instance, if your custom code does appear, but "opacity:1" is striked through, then it is because your rule is not specific enough. If this is the case, you can do something like this to make it more specific (in my browser, there is an id called aware_app on a very high level of the html document):
#aware_app .coolButton .k-sprite {
opacity: 1;
}
You can also do like this (not recommended to overuse this method):
.coolButton .k-sprite {
opacity: 1 !important;
}