aynos

Finally found the solution !

it was a bit tricky as I had to use “-webkit-” in order to change the font colors in the input button, I also had to use “!important” to override previous settings, without that, nothing is going to change !

here is what worked for me:

/* POST COMMENT BUTTON */
.highlander-dark .comments-area #respond p.form-submit input#comment-submit,input[type=submit] {
      
	border-color: #C1C1C1;
	border-width: 1.5px;
	font-weight: 600!important;
	font-size:15px!important;
	font-family:Avenir!important;
        -webkit-text-fill-color:#767676;

}

/* POST COMMENT BUTTON HOVER*/
.highlander-dark .comments-area #respond p.form-submit input#comment-submit:hover, input[type=submit]:hover {
	-webkit-text-fill-color:white!important;
	background-color:#C1C1C1!important;
        border-width:1.5px!important;
	border-color:#C1C1C1!important;
	font-weight: 500!important;
	font-size:15px!important;
	font-family:Avenir!important;
	
}

Thank you a lot @supernovia !!!!
I’ll try it later and I will tell you if it works 🙂