Below are the rules to add to the advanced css section if you want your theme to use retina ready backgrounds

1) Select Appearance -> backgrounds
2) Click choose file from the Select image option
3) Choose one of of the @X2 retina display backgrounds
4) Click Upload ( your background should appear in the preview )
5) Click Save Changes at the bottom of the page
6) Find the rule for the image you uploaded from the list Below
7) Copy the rules into your clipboard
8) Click Smartbox -> advanced
9) Paste the rule into the Extra CSS option
10) Click the Save Changes button

/* bedge_grunge_@2X.png */
body {
    background-size: 588px 375px;
}

/* lghtmesh_@2X.png */
body {
    background-size: 256px 256px;
}

/* pw_maze_black_@2X.png */
body {
    background-size: 46px 23px;
}

/* bedge_grunge_@2X.png */
body {
    background-size: 588px 375px;
}

/* bgnoise_lg_@2X.png */
body {
    background-size: 100px 100px;
}

/* noisy-grid_@2X.png */
body {
    background-size: 150px 150px;
}

/* retina_wood_@2X.png */
body {
    background-size: 512px 512px;
}

/* concrete_wall_2_@2X.png */
body {
    background-size: 597px 545px;
}

/* creampaper_@2X.png */
body {
    background-size: 158px 144px;
}

/* crisp_paper_ruffles_@2X.png */
body {
    background-size: 481px 500px;
}

/* crissXcross_@2X.png */
body {
    background-size: 512px 512px;
}

/* dark_wall_@2X.png */
body {
    background-size: 300px 300px;
}

/* dark_wood_@2X.png */
body {
    background-size: 512px 512px;
}

/* darkdenim3_@2X.png */
body {
    background-size: 420px 326px;
}

/* debut_dark_@2X.png */
body {
    background-size: 200px 200px;
}

/* first_aid_kit_@2X.png */
body {
    background-size: 99px 99px;
}

/* gradient_squares_@2X.png */
body {
    background-size: 202px 202px;
}

/* gray_jean_@2X.png */
body {
    background-size: 150px 150px;
}

/* grey_@2X.png */
body {
    background-size: 397px 322px;
}

/* grid_@2X.png */
body {
    background-size: 310px 310px;
}

/* honey_im_subtle_@2X.png */
body {
    background-size: 179px 132px;
}

/* husk_@2X.png */
body {
    background-size: 500px 500px;
}

/* ice_age_@2X.png */
body {
    background-size: 400px 400px;
}

/* irongrip_@2X.png */
body {
    background-size: 300px 301px;
}

/* little_pluses_@2X.png */
body {
    background-size: 300px 300px;
}

/* nasty_fabric_@2X.png */
body {
    background-size: 198px 200px;
}

/* subtle_stripes_@2X.png */
body {
    background-size: 40px 40px;
}

/* noise_lines_@2X.png */
body {
    background-size: 60px 59px;
}

/* project_papper_@2X.png */
body {
    background-size: 105px 105px;
}

/* purty_wood_@2X.png */
body {
    background-size: 400px 400px;
}

/* px_by_Gre3g_@2X.png */
body {
    background-size: 100px 100px;
}

/* subtle_grunge_@2X.png */
body {
    background-size: 400px 400px;
}

/* texturetastic_gray_@2X.png */
body {
    background-size: 476px 476px;
}

/* tileable_wood_texture_@2X.png */
body {
    background-size: 400px 317px;
}

/* wild_oliva_@2X.png */
body {
    background-size: 198px 200px;
}

/* wood_pattern_@2X.png */
body {
    background-size: 203px 317px;
}

/* exclusive_paper.png */
body {
    background-size: 560px 420px;
}
