Clique em "Design" e depois em "Editar HTML", após procure por:

]]></b:skin>

Acima dessa linha cole o código CSS abaixo:

nav {
        display: block;
        width: 100%;
        overflow: hidden;
}
nav ul {
        margin: 80px 0 20px 0;
        padding: .7em;
        float: left;
        list-style: none;
        background: #444;
        background: rgba(0,0,0,.2);
        -moz-border-radius: .5em;
        -webkit-border-radius: .5em;
        border-radius: .5em;
        -moz-box-shadow: 0 1px 0 rgba(255,255,255,.2), 0 2px 1px rgba(0,0,0,.8) inset;
        -webkit-box-shadow: 0 1px 0 rgba(255,255,255,.2), 0 2px 1px rgba(0,0,0,.8) inset;
        box-shadow: 0 1px 0 rgba(255,255,255,.2), 0 2px 1px rgba(0,0,0,.8) inset;
}
nav li {
        float:left;
}
nav a {
        float:left;
        padding: .8em 1.5em;
        text-decoration: none;
        color: #555;
        text-shadow: 0 1px 0 rgba(255,255,255,.5);
        font: bold 1.1em/1 'trebuchet MS', Arial, Helvetica;
        letter-spacing: 1px;
        text-transform: uppercase;
        border-width: 1px;
        border-style: solid;
        border-color: #fff #ccc #999 #eee;
        background: #c1c1c1;
        background: -moz-linear-gradient(#f5f5f5, #c1c1c1);
        background: -webkit-gradient(linear, left top, left bottom, from(#f5f5f5), to(#c1c1c1));
        background: -webkit-linear-gradient(#f5f5f5, #c1c1c1);
        background: -o-linear-gradient(#f5f5f5, #c1c1c1);
        background: -ms-linear-gradient(#f5f5f5, #c1c1c1);
        background: linear-gradient(#f5f5f5, #c1c1c1);
}
nav a:hover, nav a:focus {
        outline: 0;
        color: #fff;
        text-shadow: 0 1px 0 rgba(0,0,0,.2);
        background: #fac754;
        background: -moz-linear-gradient(#fac754, #f8ac00);
        background: -webkit-gradient(linear, left top, left bottom, from(#fac754), to(#f8ac00));
        background: -webkit-linear-gradient(#fac754, #f8ac00);
        background: -o-linear-gradient(#fac754, #f8ac00);
        background: -ms-linear-gradient(#fac754, #f8ac00);
        background: linear-gradient(#fac754, #f8ac00);
}
nav a:active {
        -moz-box-shadow: 0 0 2px 2px rgba(0,0,0,.3) inset;
        -webkit-box-shadow: 0 0 2px 2px rgba(0,0,0,.3) inset;
        box-shadow: 0 0 2px 2px rgba(0,0,0,.3) inset;
}
nav li:first-child a {
        border-left: 0;
        -moz-border-radius: 4px 0 0 4px;
        -webkit-border-radius: 4px 0 0 4px;
        border-radius: 4px 0 0 4px;
}
nav li:last-child a {
        border-right: 0;
        -moz-border-radius: 0 4px 4px 0;
        -webkit-border-radius: 0 4px 4px 0;
        border-radius: 0 4px 4px 0;
}

Feito isso, salve.

Agora acesse a guia “Design” do seu blog e clique na opção “Adicionar um gadget” –> “HTML/Javascript”. Em seguida cole o código abaixo, faça as alterações conforme o que se pede (destacado de vermelho no código) e salve.