Custom css to make button font Bold

Site URL: https://www.silverbirchwm.ca/

Hi there, I have a button on the top right of our homepage that reads:  ACCESS YOUR ACCOUNTS.

Can you please tell me how I can make the button font Bold, please? This is the custom code I have now:

div.Header-nav-inner a.Header-nav-item:last-child{
    color: #fff !important;
    padding:10px 20px !important;
    border: 1px solid #FF0000 !important;
    background-color: #FF0000 !important;
    border-radius:16px !important; 
    }
div.Header-nav-inner a.Header-nav-item:last-child:hover{
    color: #FF0000 !important;
    background-color:#fff !important;
    }

Try adding:

font-weight: 500px;

and adjust it by increasing or decreasing the 500px.

Thank you for your response.

I tried adding font-weight: 500px; a couple of different ways but I can't get it to work. I've tried it with and without adding !important; 

Maybe I'm not placing it within the existing code correctly? 

I figured it out. I added 

font-weight: bold;