Let's limit this to people active in the last 2 months.

Which skins are used most on wikidata.org,
by users active in the last 30 days
(as of 2014-12-09)?

190,monobook
18,modern
3,cologneblue
1,standard
1,simple

select count(*), substr(up_value,1,20)
from user_properties
where up_property like 'skin'
and up_user in (select rc_user
from recentchanges
where rc_timestamp > '20141109'
group by rc_user)
group by up_value
order by count(*) desc

I'd like to know in addition how many edits were done by the monobook users in that timeframe compared to the vector users.

Who is working on that?

Skin usage at the end of 2014:
190,monobook
18,modern
3,cologneblue
1,standard
1,simple
rest: vektor

total edits in 30 days: 6,9 Mio.
total edits in 30 days without bots: 1,5 Mio.
edits by monobook users in 30 days: 93000
of those 59000 edits were done by a single user