Adding 2 Buttons to Navigation Header - Bedford

Site URL: https://www.altclaim.com/

How can I make the last 2 navigation items as buttons (of separate colors) in Bedford?  I would like the text "Submit a Claim" to be a black button next to the red one.  I'm on the older version of Squarespace.

I've searched the web for over 45 min. and have struck out...found a video of needing to buy a membership to get the code.

Thanks in advance for any help!

Melissa in FL

Add to Home > Design > Custom CSS

#mainNavigation [href*="altclaim.force"] {
    background: red;
    color: white;
    padding-top: 15px;
    padding-bottom: 15px;
    border-radius: 5px;
}

Thank you Tuan.  I wanted the button black, but I changed that, yet can't get the font to be white?  It appears gray?  Can I get the text to be white to match the red button?