You can add these CSS blocks to your Thesis -> Custom CSS screen

1. Float the logo to the center

If you are using an image for your header use:

#header .wrap {
    text-align: center;
}

#thesis_header_image {
    display: inline-block;
    float: none;
}

If you are using a text header use:

#header .wrap {
    text-align: center;
}

#site_title {
    display: inline-block;
    float: none;
}

2. Change the body background color from light grey

body {background: #fff;}

3. Change the colors of the Header, Footer and Copyright Footer Areas

#navigation {background: #000;}
#footer_widgets {background: #000;}
#footer {background: #000;}

4. Change the fonts in the menus and footer areas

.menu a {
    font-family: Arial,"Helvetica Neue",Helvetica,sans-serif; 
    font-size: 16px; 
    text-shadow: none;
}

#footer, #footer_widgets .widget, #footer_widgets .widget .widget_title {
    font-family: Arial,"Helvetica Neue",Helvetica,sans-serif; 
    font-size: 16px; 
    text-shadow: none;
}