Open /admin/_menutop.php and find this part:

 <link href="variation.css" rel="stylesheet" type="text/css" title="Variation" />
        <link href="desert.css" rel="alternate stylesheet" type="text/css" title="Desert" />
        <link href="legacy.css" rel="alternate stylesheet" type="text/css" title="Legacy" />
        <?php if( is_file( dirname(__FILE__).'/custom.css' ) ) { ?>
        <link href="custom.css" rel="alternate stylesheet" type="text/css" title="Custom" />
        <?php } ?>
        <script type="text/javascript" src="styleswitcher.js"></script>

Assuming you want to only allow variation.css, change it to:

 <link href="variation.css" rel="stylesheet" type="text/css" title="Variation" />

Then you would want to find in that same file:

                <?php echo T_('Style:') ?>
                <a href="#" onclick="setActiveStyleSheet('Variation'); return false;" title="Variation (Default)">V</a>&middot;<a href="#" onclick="setActiveStyleSheet('Desert'); return false;" title="Desert">D</a>&middot;<a href="#" onclick="setActiveStyleSheet('Legacy'); return false;" title="Legacy">L</a><?php if( is_file( dirname(__FILE__).'/custom.css' ) ) { ?>&middot;<a href="#" onclick="setActiveStyleSheet('Custom'); return false;" title="Custom">C</a><?php } ?>
                &bull;

And delete that section.

Thanks for the advice. I tested it out, but unfortunately I couldn't get it to work.

After editing _menutop.php, the backoffice no longer gives you the choice of switching between different skins -- as planned -- but it doesn't preserve the chosen stylesheet at all. Everything is stripped back to the basic interface, with no style elements whatsoever.

I tried it with two different styles (I actually want to set "Legacy" as the default style) but none of them worked. :(

Thanks for the advice. I tested it out, but unfortunately I couldn't get it to work.

After editing _menutop.php, the backoffice no longer gives you the choice of switching between different skins -- as planned -- but it doesn't preserve the chosen stylesheet at all. Everything is stripped back to the basic interface, with no style elements whatsoever.

I tried it with two different styles (I actually want to set "Legacy" as the default style) but none of them worked. :(

Make sure it says rel="stylesheet", not rel="alternate stylesheet"

you're a genius. that was exactly the problem. i should have paid closer attention. i even managed to get it to default to my preffered skin -- legacy -- by substituting "legacy" for "variation."

many, many thanks. :D

<link href="legacy.css" rel="stylesheet" type="text/css" title="Legacy" />