I was really not that far!
<style>
.customSelect::selection { background: rgba(175,210,255,0.5); }
.customSelect::-moz-selection { background: rgba(175,210,255,0.5); }
</style>
If you don't specify the css color property for text color, you will only have the background in a blue color and the text keep the color assigned (as I was looking for!). You have to put the class customSelect into the div contenteditable and into the function editCSS().
<script type="text/javascript">
function editCSS(css){
document.execCommand('insertHTML', false, '<span class="customSelect" ' +
'style="'+css+'">'+document.getSelection()+'</span>');
}
</script>
HTML CODE
<div class="customSelect" contenteditable>some text</div>