Layout&Css

I have encountered an error when I decided to customize the function of registration, authorization, during this I had to correct a margin-top;

some _LoginPartial.cshtml code :

<div class="ui grid"><div class="three wide column"></div><div class="six wide column right aligned regNav" ><h3><i class="spy icon"></i><a asp-area="" asp-controller="Account" asp-action="Register">Register</a></h3></div><div class="five wide column right aligned regNav" ><h3><i class="user icon"></i> <a asp-area="" asp-controller="Account" asp-action="Login">Log in</a></h3></div><div class="two wide column"></div></div>

Then i  wrote in my main.css something like this

.regNav{
    margin-top:5px;
}

But nothing happend:(  . I rebuild my project but same problem ( 

Whats wrong?

Thanks