The browser does the interpreting. If the rule doesn’t work you’re either applying it to the incorrect element, or the custom style is not specific enough and gets overwritten by existing styling.
Try
body #frontpage { ...
Note that the #frontpage element is located on top of a different layer which already has a white (configurable) background, so any semi-transparent color you’re applying will be applied to that background, it will not make the content background semi-transparent all the way down to the site background.