[TWood]

The long answer is, install Firefox and an add-on called Firebug which will allow you to inspect the code and CSS of any website. Using Firebug on my 4.1.3 website, I get this for the CSS for the navtabs:

.navtabs li a.navtab {
    -moz-font-feature-settings: normal;
    -moz-font-language-override: normal;
    -x-system-font: none;
    background-attachment: scroll;
    background-clip: border-box;
    background-color: #7192A8;
    background-image: url("images/buttons/newbtn_middle.png");
    background-origin: padding-box;
    background-position: 0 0;
    background-repeat: repeat-x;
    background-size: auto auto;
    border-left-color-ltr-source: physical;
    border-left-color-rtl-source: physical;
    border-left-color-value: #CEDFEB;
    border-left-style-ltr-source: physical;
    border-left-style-rtl-source: physical;
    border-left-style-value: solid;
    border-left-width-ltr-source: physical;
    border-left-width-rtl-source: physical;
    border-left-width-value: 1px;
    border-top-color: #CEDFEB;
    border-top-style: solid;
    border-top-width: 1px;
    color: #FFFFFF;
    display: block;
    font-family: Arial,Calibri,Verdana,Geneva,sans-serif;
    font-size: 12px;
    font-size-adjust: none;
    font-stretch: normal;
    font-style: normal;
    font-variant: normal;
    font-weight: bold;
    line-height: 26px;
    min-width: 60px;
    text-align: center;
    text-decoration: none;
    width: auto !important;
}

Some of that is set in StyleVars, but it looks like there's a lot of it that's not. Dunno where the rest is, maybe someone else can help?