[spdesign]

For 5.1.7:

Figured it out. Follow instructions in KB here: http://kb.x-cart.com/display/XDD/Step+2+-+applying+design+changes

except with the following changes if your custom theme is a module.

In your module directory (classes/XLite/YourName/YourThemeName/View/) create AView.php with the following code

<?php
 
namespace XLite\Module\YourName\YourThemeName\View;
 
class AView extends \XLite\View\AView implements \XLite\Base\IDecorator
{
    public function getCSSFiles()
    {
        $list = parent::getCSSFiles();
 
        $list[] = 'css/yourcssfile.css';
 
        return $list;
    }  
} 

And then make sure yourcssfile.css is in /skins/YourThemeName/default/en/css/

Probably pretty basic and unnecessary, but just in case others are less experienced with XCart 5 like me and need it really spelled out.