ext.echo.alert: Restore orange background on Monobook

Core styles for Monobook include high-specificity background: transparent;
rule for #p-personal li a, we need to match it to set our background.

Also change hover behavior: switch to a deeper orange instead of
default white, similarly to how the badge already behaves.

Partially reverts I682182fe.

Bug: 56214
Change-Id: I9f343264c395ecf09c1e34e03d208ec2119fb622
(cherry picked from commit dc1443f57f63e9f5c8609cd9450a6f074ad0078b)