Bring back dark theme code block border
Bring back dark theme code block border

This fixes https://github.com/vector-im/riot-web/issues/5013 and https://github.com/vector-im/riot-web/issues/4416 by only showing the border when the code block is hovered, and that the border is actually visible. The !important is needed to override the default border behaviour.

Signed-off-by: Travis Ralston <f36f6a6abc4b55589c96c71fceedf83e596b8f33@gmail.com>
vector-im/riot-web,vector-im/riot-web,martindale/vector,vector-im/riot-web,vector-im/vector-web,vector-im/riot-web,vector-im/riot-web,vector-im/vector-web,vector-im/riot-web,vector-im/vector-web,martindale/vector,vector-im/vector-web,vector-im/vector-web