Allow custom css

[joshuarestivo]

CRM should allow custom css to be applied. Ideally, in keeping with the Frappe approach, it would honor css supplied via a custom application so that it can be themed as needed.

Note that I'm not necessarily asking for someone else to add this. Rather, I'd like to know if it would be accepted as an upstream contribution or if there's some fundamental misgiving about allowing a feature like this?