Make archive file icon shadows visible in dark mode.

Make archive file icon shadows visible in dark mode.

On a black background, the black icon shadows are invisible.  This is
not a huge problem but we can use CSS rules to make them visible again.

To do this, the SVG embedding is adjusted to automatically add classes
based on the filename in order to differentiate between the various
images, and additional classes are added to the icons to enable
selectors to match the shadows.

The results seem OK.