If you’re new to Sandbox and applying CSS skins, this will hopefully guide you. Just click on Presentation > Themes. Select the Sandbox version the skin has been coded for.

Sandbox:
If the skin has been coded for Sandbox, select it by clicking on its thumbnail, activate the theme and then click on Sandbox Skins. From the Available Skins list, select No Stylesheet (its thumbnail should read “Preview not available”). Once you have selected it, click on Edit CSS. Paste the CSS code in the CSS Sylesheet Editor, check the Start from scratch and just use this radio button. Save, et voilà.

Sandbox-10:
If the skin has been coded for Sandbox-10, click on Presentation > Themes. Select Sandbox-10 and activate it. Once it’s been activated, click on Edit CSS, paste the CSS code in the CSS Stylesheet Editor; check the Start from scratch and just use this radio button. Save, et voilà.

Resetting or Changing Themes
If later you want to change themes or reset your Sandbox theme, just delete the code in the CSS Stylesheet Editor, check the Add to existing CSS (View stylesheet) radio button and Save.

Hope this helps.