You could use a CSS pre-processor to do your skinning. With Sass you can do something similar to this:
_theme1.scss:
$theme-primary-color: #a00;
$theme-secondary-color: #d00;
// etc.
_theme2.scss:
$theme-primary-color: #666;
$theme-secondary-color: #ccc;
// etc.
styles.scss:
// import whichever theme you want to use
@import 'theme2';
-webkit-box-shadow: inset 0px 0px 2px $theme-primary-color;
-moz-box-shadow: inset 0px 0px 2px $theme-primary-color;
If it's not site wide theming but class based theming you need, then you can do this: http://codepen.io/jjenzz/pen/EaAzo