<{* pick the css directory you want to use for the dark theme on the bottom line.
That theme will be used for media="(prefers-color-scheme: dark)"

Leave blank to not use the prefers-color-scheme media query, and just use
the theme selected in xswatchCss.tpl

The subset of valid themes that are considered dark are:
 css-cyborg    css-darkly    css-slate     css-solar     css-superhero
*}>
css-slate
