css: fix debian openlogo background-size for chrom* based browsers

css: fix debian openlogo background-size for chrom* based browsers

The debian one higher than wide and it seems, contrary to my belief,
the background-size is not taken for both, height and width if only
one param is set, but rather the second paramet height then defaults
to `auto` which Firefox and Chromium handle different in this case.

Set both to make this fixed.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>